/* ==========================================================================
   CSS Custom Properties (Design Tokens)
   ========================================================================== */
:root {
  /* Colors - Dark Theme Default */
  --color-bg-primary: #0d1117;
  --color-bg-secondary: #161b22;
  --color-bg-tertiary: #21262d;
  --color-bg-hover: #30363d;
  --color-bg-active: #238636;

  --color-border-primary: #30363d;
  --color-border-secondary: #484f58;
  --color-border-focus: #58a6ff;

  --color-text-primary: #f0f6fc;
  --color-text-secondary: #8b949e;
  --color-text-muted: #6e7681;
  --color-text-inverse: #0d1117;

  --color-accent-primary: #58a6ff;
  --color-accent-secondary: #a371f7;
  --color-accent-success: #3fb950;
  --color-accent-warning: #d29922;
  --color-accent-danger: #f85149;

  --color-accent-primary-dim: rgba(88, 166, 255, 0.15);
  --color-accent-secondary-dim: rgba(163, 113, 247, 0.15);
  --color-accent-success-dim: rgba(63, 185, 80, 0.15);
  --color-accent-warning-dim: rgba(210, 153, 34, 0.15);
  --color-accent-danger-dim: rgba(248, 81, 73, 0.15);

  /* Simulation-specific colors */
  --color-sim-pendulum: #58a6ff;
  --color-sim-projectile: #f78166;
  --color-sim-spring: #a371f7;
  --color-sim-orbits: #7ee787;
  --color-sim-double-pendulum: #ffa657;

  /* Spacing */
  --space-xs: 4px;
  --space-sm: 8px;
  --space-md: 16px;
  --space-lg: 24px;
  --space-xl: 32px;
  --space-2xl: 48px;

  /* Typography */
  --font-family-base: -apple-system, BlinkMacSystemFont, 'Segoe UI', 'Noto Sans', Helvetica, Arial, sans-serif;
  --font-family-mono: 'SF Mono', 'Fira Code', 'Fira Mono', Menlo, Consolas, monospace;
  --font-size-xs: 11px;
  --font-size-sm: 12px;
  --font-size-base: 14px;
  --font-size-md: 16px;
  --font-size-lg: 18px;
  --font-size-xl: 20px;
  --font-size-2xl: 24px;
  --font-size-3xl: 32px;
  --font-size-4xl: 40px;
  --font-weight-normal: 400;
  --font-weight-medium: 500;
  --font-weight-semibold: 600;
  --font-weight-bold: 700;

  /* Border Radius */
  --radius-sm: 4px;
  --radius-md: 8px;
  --radius-lg: 12px;
  --radius-xl: 16px;
  --radius-full: 9999px;

  /* Shadows */
  --shadow-sm: 0 1px 2px rgba(0, 0, 0, 0.3);
  --shadow-md: 0 4px 12px rgba(0, 0, 0, 0.4);
  --shadow-lg: 0 8px 24px rgba(0, 0, 0, 0.5);
  --shadow-xl: 0 16px 48px rgba(0, 0, 0, 0.6);

  /* Transitions */
  --transition-fast: 150ms ease;
  --transition-base: 250ms ease;
  --transition-slow: 350ms ease;

  /* Z-Indices */
  --z-dropdown: 100;
  --z-sticky: 200;
  --z-modal: 300;
  --z-popover: 400;
  --z-tooltip: 500;

  /* Layout */
  --header-height: 64px;
  --panel-width: 320px;
  --panel-collapsed-width: 48px;
  --info-panel-height: 280px;
}

/* Light theme (optional - can be toggled) */
[data-theme="light"] {
  --color-bg-primary: #ffffff;
  --color-bg-secondary: #f6f8fa;
  --color-bg-tertiary: #f0f4f8;
  --color-bg-hover: #e1e4e8;
  --color-bg-active: #0969da;

  --color-border-primary: #d0d7de;
  --color-border-secondary: #a8b0ba;
  --color-border-focus: #0969da;

  --color-text-primary: #1f2328;
  --color-text-secondary: #57606a;
  --color-text-muted: #6e7781;
  --color-text-inverse: #ffffff;

  --color-accent-primary: #0969da;
  --color-accent-secondary: #8250df;
  --color-accent-success: #1a7f37;
  --color-accent-warning: #9a6700;
  --color-accent-danger: #cf222e;

  --shadow-sm: 0 1px 2px rgba(0, 0, 0, 0.08);
  --shadow-md: 0 4px 12px rgba(0, 0, 0, 0.1);
  --shadow-lg: 0 8px 24px rgba(0, 0, 0, 0.12);
  --shadow-xl: 0 16px 48px rgba(0, 0, 0, 0.15);
}

/* ==========================================================================
   Reset & Base Styles
   ========================================================================== */
*, *::before, *::after {
  box-sizing: border-box;
  margin: 0;
  padding: 0;
}

html {
  font-size: 14px;
  -webkit-font-smoothing: antialiased;
  -moz-osx-font-smoothing: grayscale;
}

body {
  font-family: var(--font-family-base);
  font-size: var(--font-size-base);
  line-height: 1.5;
  color: var(--color-text-primary);
  background-color: var(--color-bg-primary);
  min-height: 100vh;
  overflow-x: hidden;
}

a {
  color: var(--color-accent-primary);
  text-decoration: none;
  transition: color var(--transition-fast);
}

a:hover {
  color: var(--color-accent-secondary);
}

a:focus-visible,
button:focus-visible,
input:focus-visible,
select:focus-visible,
textarea:focus-visible {
  outline: 2px solid var(--color-border-focus);
  outline-offset: 2px;
}

img, video, canvas {
  max-width: 100%;
  height: auto;
  display: block;
}

button {
  font-family: inherit;
  font-size: inherit;
  cursor: pointer;
  border: none;
  background: none;
}

input, select, textarea {
  font-family: inherit;
  font-size: inherit;
  color: inherit;
}

/* Scrollbar */
::-webkit-scrollbar {
  width: 8px;
  height: 8px;
}
::-webkit-scrollbar-track {
  background: var(--color-bg-primary);
}
::-webkit-scrollbar-thumb {
  background: var(--color-border-secondary);
  border-radius: var(--radius-full);
}
::-webkit-scrollbar-thumb:hover {
  background: var(--color-text-muted);
}

/* Selection */
::selection {
  background: var(--color-accent-primary-dim);
  color: var(--color-text-primary);
}

/* ==========================================================================
   Layout
   ========================================================================== */
.app {
  display: flex;
  flex-direction: column;
  min-height: 100vh;
}

.main-content {
  flex: 1;
  display: flex;
  flex-direction: column;
}

/* ==========================================================================
   Header
   ========================================================================== */
.header {
  position: fixed;
  top: 0;
  left: 0;
  right: 0;
  height: var(--header-height);
  background: rgba(13, 17, 23, 0.95);
  backdrop-filter: blur(20px);
  border-bottom: 1px solid var(--color-border-primary);
  z-index: var(--z-sticky);
  display: flex;
  align-items: center;
  justify-content: space-between;
  padding: 0 var(--space-lg);
}

.header-left {
  display: flex;
  align-items: center;
  gap: var(--space-lg);
}

.logo {
  display: flex;
  align-items: center;
  gap: var(--space-sm);
  font-size: var(--font-size-xl);
  font-weight: var(--font-weight-bold);
  color: var(--color-text-primary);
  text-decoration: none;
}

.logo-icon {
  width: 32px;
  height: 32px;
  background: linear-gradient(135deg, var(--color-accent-primary), var(--color-accent-secondary));
  border-radius: var(--radius-md);
  display: flex;
  align-items: center;
  justify-content: center;
  font-size: var(--font-size-lg);
}

.nav {
  display: flex;
  gap: var(--space-xs);
}

.nav-link {
  display: flex;
  align-items: center;
  gap: var(--space-sm);
  padding: var(--space-sm) var(--space-md);
  border-radius: var(--radius-md);
  color: var(--color-text-secondary);
  font-weight: var(--font-weight-medium);
  font-size: var(--font-size-sm);
  transition: all var(--transition-fast);
}

.nav-link:hover,
.nav-link.active {
  color: var(--color-text-primary);
  background: var(--color-bg-tertiary);
}

.nav-link.active {
  color: var(--color-accent-primary);
}

.nav-icon {
  width: 18px;
  height: 18px;
}

.header-right {
  display: flex;
  align-items: center;
  gap: var(--space-md);
}

.theme-toggle {
  width: 40px;
  height: 40px;
  border-radius: var(--radius-full);
  display: flex;
  align-items: center;
  justify-content: center;
  color: var(--color-text-secondary);
  transition: all var(--transition-fast);
}

.theme-toggle:hover {
  background: var(--color-bg-tertiary);
  color: var(--color-text-primary);
}

.sim-selector {
  position: relative;
}

.sim-select-btn {
  display: flex;
  align-items: center;
  gap: var(--space-sm);
  padding: var(--space-sm) var(--space-md);
  background: var(--color-bg-tertiary);
  border: 1px solid var(--color-border-primary);
  border-radius: var(--radius-md);
  color: var(--color-text-primary);
  font-size: var(--font-size-sm);
  font-weight: var(--font-weight-medium);
  transition: all var(--transition-fast);
}

.sim-select-btn:hover {
  border-color: var(--color-border-secondary);
  background: var(--color-bg-hover);
}

.sim-select-btn .sim-icon {
  font-size: var(--font-size-md);
}

.sim-select-btn .chevron {
  width: 16px;
  height: 16px;
  transition: transform var(--transition-fast);
}

.sim-select-btn[aria-expanded="true"] .chevron {
  transform: rotate(180deg);
}

.sim-dropdown {
  position: absolute;
  top: calc(100% + var(--space-xs));
  right: 0;
  min-width: 280px;
  background: var(--color-bg-secondary);
  border: 1px solid var(--color-border-primary);
  border-radius: var(--radius-lg);
  box-shadow: var(--shadow-lg);
  padding: var(--space-sm);
  opacity: 0;
  visibility: hidden;
  transform: translateY(-8px);
  transition: all var(--transition-fast);
  z-index: var(--z-dropdown);
}

.sim-dropdown.open {
  opacity: 1;
  visibility: visible;
  transform: translateY(0);
}

.sim-dropdown-item {
  display: flex;
  align-items: center;
  gap: var(--space-md);
  padding: var(--space-md);
  border-radius: var(--radius-md);
  color: var(--color-text-primary);
  text-decoration: none;
  transition: all var(--transition-fast);
}

.sim-dropdown-item:hover {
  background: var(--color-bg-tertiary);
}

.sim-dropdown-item .sim-icon {
  font-size: var(--font-size-xl);
  width: 40px;
  height: 40px;
  display: flex;
  align-items: center;
  justify-content: center;
  background: var(--color-bg-tertiary);
  border-radius: var(--radius-md);
}

.sim-dropdown-item.active {
  background: var(--color-accent-primary-dim);
  border: 1px solid var(--color-accent-primary);
}

.sim-dropdown-info {
  flex: 1;
  min-width: 0;
}

.sim-dropdown-title {
  font-weight: var(--font-weight-semibold);
  font-size: var(--font-size-sm);
  white-space: nowrap;
  overflow: hidden;
  text-overflow: ellipsis;
}

.sim-dropdown-desc {
  font-size: var(--font-size-xs);
  color: var(--color-text-muted);
  white-space: nowrap;
  overflow: hidden;
  text-overflow: ellipsis;
}

/* ==========================================================================
   Hero Section (Home Page)
   ========================================================================== */
.hero {
  display: flex;
  flex-direction: column;
  align-items: center;
  justify-content: center;
  min-height: 100vh;
  padding: calc(var(--header-height) + var(--space-2xl)) var(--space-lg) var(--space-2xl);
  text-align: center;
  position: relative;
  overflow: hidden;
}

.hero::before {
  content: '';
  position: absolute;
  top: 0;
  left: 0;
  right: 0;
  bottom: 0;
  background:
    radial-gradient(ellipse 80% 50% at 50% -20%, rgba(88, 166, 255, 0.15), transparent),
    radial-gradient(ellipse 60% 40% at 80% 100%, rgba(163, 113, 247, 0.1), transparent),
    radial-gradient(ellipse 50% 30% at 20% 0%, rgba(126, 231, 135, 0.08), transparent);
  pointer-events: none;
}

.hero-content {
  position: relative;
  z-index: 1;
  max-width: 800px;
}

.hero-badge {
  display: inline-flex;
  align-items: center;
  gap: var(--space-sm);
  padding: var(--space-sm) var(--space-lg);
  background: var(--color-accent-primary-dim);
  border: 1px solid var(--color-accent-primary);
  border-radius: var(--radius-full);
  color: var(--color-accent-primary);
  font-size: var(--font-size-sm);
  font-weight: var(--font-weight-medium);
  margin-bottom: var(--space-lg);
  animation: fadeInUp 0.6s ease;
}

.hero-title {
  font-size: clamp(var(--font-size-3xl), 6vw, var(--font-size-4xl));
  font-weight: var(--font-weight-bold);
  line-height: 1.1;
  margin-bottom: var(--space-md);
  background: linear-gradient(135deg, var(--color-text-primary), var(--color-accent-primary));
  -webkit-background-clip: text;
  -webkit-text-fill-color: transparent;
  background-clip: text;
  animation: fadeInUp 0.6s ease 0.1s both;
}

.hero-subtitle {
  font-size: var(--font-size-lg);
  color: var(--color-text-secondary);
  margin-bottom: var(--space-xl);
  max-width: 600px;
  margin-left: auto;
  margin-right: auto;
  animation: fadeInUp 0.6s ease 0.2s both;
}

.hero-actions {
  display: flex;
  gap: var(--space-md);
  justify-content: center;
  flex-wrap: wrap;
  animation: fadeInUp 0.6s ease 0.3s both;
}

.hero-stats {
  display: flex;
  gap: var(--space-2xl);
  margin-top: var(--space-2xl);
  padding-top: var(--space-xl);
  border-top: 1px solid var(--color-border-primary);
  animation: fadeInUp 0.6s ease 0.4s both;
}

.hero-stat {
  text-align: center;
}

.hero-stat-value {
  font-size: var(--font-size-3xl);
  font-weight: var(--font-weight-bold);
  color: var(--color-accent-primary);
  line-height: 1;
}

.hero-stat-label {
  font-size: var(--font-size-sm);
  color: var(--color-text-muted);
  margin-top: var(--space-xs);
}

@keyframes fadeInUp {
  from {
    opacity: 0;
    transform: translateY(20px);
  }
  to {
    opacity: 1;
    transform: translateY(0);
  }
}

/* ==========================================================================
   Simulations Grid (Home Page)
   ========================================================================== */
.simulations-section {
  padding: var(--space-2xl) var(--space-lg);
  max-width: 1200px;
  margin: 0 auto;
  width: 100%;
}

.section-header {
  text-align: center;
  margin-bottom: var(--space-xl);
}

.section-title {
  font-size: var(--font-size-2xl);
  font-weight: var(--font-weight-bold);
  margin-bottom: var(--space-sm);
}

.section-subtitle {
  color: var(--color-text-secondary);
  font-size: var(--font-size-md);
}

.simulations-grid {
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(300px, 1fr));
  gap: var(--space-lg);
}

.sim-card {
  background: var(--color-bg-secondary);
  border: 1px solid var(--color-border-primary);
  border-radius: var(--radius-xl);
  padding: var(--space-xl);
  transition: all var(--transition-base);
  position: relative;
  overflow: hidden;
}

.sim-card::before {
  content: '';
  position: absolute;
  top: 0;
  left: 0;
  right: 0;
  height: 3px;
  background: var(--sim-color, var(--color-accent-primary));
  transform: scaleX(0);
  transform-origin: left;
  transition: transform var(--transition-base);
}

.sim-card:hover {
  border-color: var(--color-border-secondary);
  transform: translateY(-4px);
  box-shadow: var(--shadow-lg);
}

.sim-card:hover::before {
  transform: scaleX(1);
}

.sim-card-header {
  display: flex;
  align-items: flex-start;
  gap: var(--space-md);
  margin-bottom: var(--space-md);
}

.sim-card-icon {
  width: 56px;
  height: 56px;
  border-radius: var(--radius-lg);
  background: var(--sim-color-dim, var(--color-accent-primary-dim));
  display: flex;
  align-items: center;
  justify-content: center;
  font-size: var(--font-size-2xl);
  flex-shrink: 0;
}

.sim-card-title {
  font-size: var(--font-size-lg);
  font-weight: var(--font-weight-semibold);
  flex: 1;
}

.sim-card-desc {
  color: var(--color-text-secondary);
  font-size: var(--font-size-sm);
  line-height: 1.6;
  margin-bottom: var(--space-lg);
}

.sim-card-concepts {
  display: flex;
  flex-wrap: wrap;
  gap: var(--space-sm);
  margin-bottom: var(--space-lg);
}

.concept-tag {
  font-size: var(--font-size-xs);
  padding: var(--space-xs) var(--space-sm);
  background: var(--color-bg-tertiary);
  border: 1px solid var(--color-border-primary);
  border-radius: var(--radius-full);
  color: var(--color-text-secondary);
}

.sim-card-footer {
  display: flex;
  align-items: center;
  justify-content: space-between;
  padding-top: var(--space-md);
  border-top: 1px solid var(--color-border-primary);
}

.sim-card-link {
  display: inline-flex;
  align-items: center;
  gap: var(--space-sm);
  font-weight: var(--font-weight-medium);
  font-size: var(--font-size-sm);
  color: var(--color-accent-primary);
}

.sim-card-link:hover {
  gap: var(--space-md);
}

.sim-card-link svg {
  transition: transform var(--transition-fast);
}

.sim-card-link:hover svg {
  transform: translateX(2px);
}

.sim-badge {
  font-size: var(--font-size-xs);
  padding: var(--space-xs) var(--space-sm);
  border-radius: var(--radius-full);
  font-weight: var(--font-weight-medium);
}

.sim-badge.new {
  background: var(--color-accent-success-dim);
  color: var(--color-accent-success);
}

.sim-badge.popular {
  background: var(--color-accent-warning-dim);
  color: var(--color-accent-warning);
}

/* Simulation-specific colors */
.sim-card[data-sim="pendulum"] { --sim-color: var(--color-sim-pendulum); --sim-color-dim: var(--color-accent-primary-dim); }
.sim-card[data-sim="projectile"] { --sim-color: var(--color-sim-projectile); --sim-color-dim: var(--color-accent-danger-dim); }
.sim-card[data-sim="spring"] { --sim-color: var(--color-sim-spring); --sim-color-dim: var(--color-accent-secondary-dim); }
.sim-card[data-sim="orbits"] { --sim-color: var(--color-sim-orbits); --sim-color-dim: var(--color-accent-success-dim); }
.sim-card[data-sim="double-pendulum"] { --sim-color: var(--color-sim-double-pendulum); --sim-color-dim: var(--color-accent-warning-dim); }

/* ==========================================================================
   Page Header (Simulation Pages)
   ========================================================================== */
.page-header {
  padding: calc(var(--header-height) + var(--space-xl)) var(--space-lg) var(--space-xl);
  max-width: 1400px;
  margin: 0 auto;
  width: 100%;
}

.page-header-content {
  display: flex;
  align-items: flex-start;
  gap: var(--space-xl);
  flex-wrap: wrap;
}

.page-header-icon {
  width: 80px;
  height: 80px;
  border-radius: var(--radius-xl);
  background: var(--sim-color-dim, var(--color-accent-primary-dim));
  display: flex;
  align-items: center;
  justify-content: center;
  font-size: var(--font-size-3xl);
  flex-shrink: 0;
}

.page-header-text {
  flex: 1;
  min-width: 200px;
}

.page-header-title {
  font-size: var(--font-size-3xl);
  font-weight: var(--font-weight-bold);
  margin-bottom: var(--space-xs);
}

.page-header-subtitle {
  color: var(--color-text-secondary);
  font-size: var(--font-size-md);
  margin-bottom: var(--space-md);
}

.page-header-concepts {
  display: flex;
  flex-wrap: wrap;
  gap: var(--space-sm);
}

.page-header-concepts .concept-tag {
  background: var(--color-bg-tertiary);
  border-color: var(--color-border-primary);
}

/* ==========================================================================
   Simulation Layout
   ========================================================================== */
.simulation-layout {
  display: flex;
  flex: 1;
  max-width: 1400px;
  margin: 0 auto;
  width: 100%;
  position: relative;
}

.simulation-main {
  flex: 1;
  display: flex;
  flex-direction: column;
  position: relative;
  min-height: calc(100vh - var(--header-height));
}

/* ==========================================================================
   Control Panel (Top Left)
   ========================================================================== */
.control-panel {
  position: fixed;
  top: calc(var(--header-height) + var(--space-md));
  left: var(--space-md);
  width: var(--panel-width);
  max-height: calc(100vh - var(--header-height) - var(--info-panel-height) - var(--space-md) * 3);
  background: var(--color-bg-secondary);
  border: 1px solid var(--color-border-primary);
  border-radius: var(--radius-lg);
  box-shadow: var(--shadow-lg);
  z-index: var(--z-sticky);
  display: flex;
  flex-direction: column;
  overflow: hidden;
  transition: all var(--transition-base);
}

.control-panel.collapsed {
  width: var(--panel-collapsed-width);
}

.control-panel.collapsed .panel-content,
.control-panel.collapsed .panel-header h2 {
  display: none;
}

.control-panel.collapsed .panel-toggle svg {
  transform: rotate(180deg);
}

.panel-header {
  display: flex;
  align-items: center;
  justify-content: space-between;
  padding: var(--space-md);
  border-bottom: 1px solid var(--color-border-primary);
  background: var(--color-bg-tertiary);
}

.panel-header h2 {
  font-size: var(--font-size-md);
  font-weight: var(--font-weight-semibold);
}

.panel-toggle {
  width: 32px;
  height: 32px;
  border-radius: var(--radius-md);
  display: flex;
  align-items: center;
  justify-content: center;
  color: var(--color-text-secondary);
  transition: all var(--transition-fast);
}

.panel-toggle:hover {
  background: var(--color-bg-hover);
  color: var(--color-text-primary);
}

.panel-content {
  flex: 1;
  overflow-y: auto;
  padding: var(--space-md);
}

.control-group {
  margin-bottom: var(--space-lg);
}

.control-group:last-child {
  margin-bottom: 0;
}

.control-group h3 {
  font-size: var(--font-size-sm);
  font-weight: var(--font-weight-semibold);
  color: var(--color-text-secondary);
  text-transform: uppercase;
  letter-spacing: 0.05em;
  margin-bottom: var(--space-md);
}

/* Variables Grid */
.variables-grid {
  display: flex;
  flex-direction: column;
  gap: var(--space-md);
}

.variable-control {
  display: flex;
  flex-direction: column;
  gap: var(--space-xs);
}

.variable-label {
  display: flex;
  align-items: center;
  justify-content: space-between;
  font-size: var(--font-size-sm);
  font-weight: var(--font-weight-medium);
}

.variable-value {
  font-family: var(--font-family-mono);
  font-size: var(--font-size-sm);
  color: var(--color-accent-primary);
  background: var(--color-accent-primary-dim);
  padding: 2px 8px;
  border-radius: var(--radius-sm);
  min-width: 50px;
  text-align: right;
}

.variable-slider {
  -webkit-appearance: none;
  appearance: none;
  width: 100%;
  height: 6px;
  border-radius: var(--radius-full);
  background: var(--color-bg-tertiary);
  outline: none;
  cursor: pointer;
}

.variable-slider::-webkit-slider-thumb {
  -webkit-appearance: none;
  appearance: none;
  width: 18px;
  height: 18px;
  border-radius: var(--radius-full);
  background: var(--color-accent-primary);
  cursor: pointer;
  transition: transform var(--transition-fast), box-shadow var(--transition-fast);
  box-shadow: 0 2px 4px rgba(0, 0, 0, 0.3);
}

.variable-slider::-webkit-slider-thumb:hover {
  transform: scale(1.15);
  box-shadow: 0 4px 12px rgba(88, 166, 255, 0.4);
}

.variable-slider::-moz-range-thumb {
  width: 18px;
  height: 18px;
  border: none;
  border-radius: var(--radius-full);
  background: var(--color-accent-primary);
  cursor: pointer;
}

.variable-input-row {
  display: flex;
  gap: var(--space-sm);
  align-items: center;
}

.variable-input {
  flex: 1;
  padding: var(--space-sm);
  background: var(--color-bg-tertiary);
  border: 1px solid var(--color-border-primary);
  border-radius: var(--radius-md);
  font-family: var(--font-family-mono);
  font-size: var(--font-size-sm);
  transition: border-color var(--transition-fast);
}

.variable-input:focus {
  border-color: var(--color-border-focus);
}

.variable-unit {
  color: var(--color-text-muted);
  font-size: var(--font-size-sm);
  min-width: 40px;
  text-align: right;
}

/* Data Display */
.data-display {
  display: grid;
  grid-template-columns: 1fr 1fr;
  gap: var(--space-sm);
}

.data-item {
  display: flex;
  flex-direction: column;
  gap: 2px;
  padding: var(--space-sm);
  background: var(--color-bg-tertiary);
  border-radius: var(--radius-md);
}

.data-label {
  font-size: var(--font-size-xs);
  color: var(--color-text-muted);
  text-transform: uppercase;
  letter-spacing: 0.05em;
}

.data-value {
  font-family: var(--font-family-mono);
  font-size: var(--font-size-md);
  font-weight: var(--font-weight-semibold);
  color: var(--color-text-primary);
}

/* Graph Container */
.graph-container {
  position: relative;
  height: 150px;
  background: var(--color-bg-primary);
  border-radius: var(--radius-md);
  border: 1px solid var(--color-border-primary);
}

.graph-controls {
  display: flex;
  gap: var(--space-md);
  margin-top: var(--space-sm);
  flex-wrap: wrap;
}

.checkbox-label {
  display: flex;
  align-items: center;
  gap: var(--space-xs);
  font-size: var(--font-size-xs);
  color: var(--color-text-secondary);
  cursor: pointer;
}

.checkbox-label input[type="checkbox"] {
  width: 14px;
  height: 14px;
  accent-color: var(--color-accent-primary);
}

/* ==========================================================================
   Canvas Container
   ========================================================================== */
.canvas-container {
  flex: 1;
  position: relative;
  background: var(--color-bg-primary);
  overflow: hidden;
}

#three-canvas {
  width: 100%;
  height: 100%;
  display: block;
}

.canvas-overlay {
  position: absolute;
  top: 0;
  left: 0;
  right: 0;
  bottom: 0;
  pointer-events: none;
  display: flex;
  align-items: center;
  justify-content: center;
}

.loading-overlay {
  display: flex;
  flex-direction: column;
  align-items: center;
  gap: var(--space-md);
  color: var(--color-text-secondary);
}

.spinner {
  width: 48px;
  height: 48px;
  border: 3px solid var(--color-border-primary);
  border-top-color: var(--color-accent-primary);
  border-radius: 50%;
  animation: spin 1s linear infinite;
}

@keyframes spin {
  to { transform: rotate(360deg); }
}

/* ==========================================================================
   Info Panel (Bottom)
   ========================================================================== */
.info-panel {
  position: fixed;
  bottom: 0;
  left: 0;
  right: 0;
  height: var(--info-panel-height);
  background: var(--color-bg-secondary);
  border-top: 1px solid var(--color-border-primary);
  box-shadow: 0 -4px 20px rgba(0, 0, 0, 0.3);
  z-index: var(--z-sticky);
  display: flex;
  flex-direction: column;
}

.info-tabs {
  display: flex;
  border-bottom: 1px solid var(--color-border-primary);
  background: var(--color-bg-tertiary);
  padding: 0 var(--space-md);
}

.info-tab {
  padding: var(--space-sm) var(--space-md);
  font-size: var(--font-size-sm);
  font-weight: var(--font-weight-medium);
  color: var(--color-text-secondary);
  border-bottom: 2px solid transparent;
  margin-bottom: -1px;
  transition: all var(--transition-fast);
}

.info-tab:hover {
  color: var(--color-text-primary);
}

.info-tab.active {
  color: var(--color-accent-primary);
  border-bottom-color: var(--color-accent-primary);
}

.info-panels {
  flex: 1;
  overflow-y: auto;
  padding: var(--space-md);
}

.info-panel-content {
  display: none;
  animation: fadeIn 0.2s ease;
}

.info-panel-content.active {
  display: block;
}

@keyframes fadeIn {
  from { opacity: 0; transform: translateY(4px); }
  to { opacity: 1; transform: translateY(0); }
}

/* Concepts Panel */
.concepts-list {
  display: flex;
  flex-direction: column;
  gap: var(--space-md);
}

.concept-item {
  padding: var(--space-md);
  background: var(--color-bg-tertiary);
  border-radius: var(--radius-md);
  border: 1px solid var(--color-border-primary);
}

.concept-title {
  font-weight: var(--font-weight-semibold);
  margin-bottom: var(--space-xs);
}

.concept-desc {
  color: var(--color-text-secondary);
  font-size: var(--font-size-sm);
  line-height: 1.6;
}

/* Equations Panel */
.equations-list {
  display: flex;
  flex-direction: column;
  gap: var(--space-md);
}

.equation-item {
  background: var(--color-bg-primary);
  border: 1px solid var(--color-border-primary);
  border-radius: var(--radius-md);
  padding: var(--space-md);
}

.equation-title {
  font-weight: var(--font-weight-semibold);
  margin-bottom: var(--space-sm);
  color: var(--color-accent-primary);
}

.equation-formula {
  font-family: var(--font-family-mono);
  background: var(--color-bg-tertiary);
  padding: var(--space-sm);
  border-radius: var(--radius-sm);
  overflow-x: auto;
  font-size: var(--font-size-sm);
  line-height: 1.5;
}

.equation-vars {
  margin-top: var(--space-sm);
  font-size: var(--font-size-xs);
  color: var(--color-text-muted);
}

/* Data Panel */
.data-table {
  width: 100%;
  border-collapse: collapse;
  font-size: var(--font-size-sm);
}

.data-table th,
.data-table td {
  padding: var(--space-sm) var(--space-md);
  text-align: left;
  border-bottom: 1px solid var(--color-border-primary);
}

.data-table th {
  color: var(--color-text-secondary);
  font-weight: var(--font-weight-medium);
  font-size: var(--font-size-xs);
  text-transform: uppercase;
  letter-spacing: 0.05em;
}

.data-table td {
  font-family: var(--font-family-mono);
}

.data-table tr:last-child td {
  border-bottom: none;
}

.data-table tr:hover td {
  background: var(--color-bg-tertiary);
}

/* ==========================================================================
   Introduction Modal
   ========================================================================== */
.modal-overlay {
  position: fixed;
  inset: 0;
  background: rgba(0, 0, 0, 0.8);
  backdrop-filter: blur(4px);
  z-index: var(--z-modal);
  display: flex;
  align-items: center;
  justify-content: center;
  padding: var(--space-lg);
  opacity: 0;
  visibility: hidden;
  transition: all var(--transition-base);
}

.modal-overlay.open {
  opacity: 1;
  visibility: visible;
}

.modal {
  background: var(--color-bg-secondary);
  border: 1px solid var(--color-border-primary);
  border-radius: var(--radius-xl);
  max-width: 700px;
  width: 100%;
  max-height: 90vh;
  overflow: hidden;
  display: flex;
  flex-direction: column;
  transform: scale(0.95) translateY(20px);
  transition: transform var(--transition-base);
}

.modal-overlay.open .modal {
  transform: scale(1) translateY(0);
}

.modal-header {
  display: flex;
  align-items: flex-start;
  justify-content: space-between;
  padding: var(--space-xl);
  border-bottom: 1px solid var(--color-border-primary);
}

.modal-icon {
  width: 64px;
  height: 64px;
  border-radius: var(--radius-lg);
  background: var(--sim-color-dim, var(--color-accent-primary-dim));
  display: flex;
  align-items: center;
  justify-content: center;
  font-size: var(--font-size-2xl);
  flex-shrink: 0;
}

.modal-title-group {
  flex: 1;
  margin-left: var(--space-md);
  min-width: 0;
}

.modal-title {
  font-size: var(--font-size-2xl);
  font-weight: var(--font-weight-bold);
  margin-bottom: var(--space-xs);
}

.modal-subtitle {
  color: var(--color-text-secondary);
}

.modal-close {
  width: 40px;
  height: 40px;
  border-radius: var(--radius-md);
  display: flex;
  align-items: center;
  justify-content: center;
  color: var(--color-text-secondary);
  flex-shrink: 0;
}

.modal-close:hover {
  background: var(--color-bg-tertiary);
  color: var(--color-text-primary);
}

.modal-body {
  flex: 1;
  overflow-y: auto;
  padding: var(--space-xl);
}

.modal-section {
  margin-bottom: var(--space-xl);
}

.modal-section:last-child {
  margin-bottom: 0;
}

.modal-section-title {
  font-size: var(--font-size-md);
  font-weight: var(--font-weight-semibold);
  margin-bottom: var(--space-md);
  color: var(--color-accent-primary);
}

.modal-concepts {
  display: flex;
  flex-direction: column;
  gap: var(--space-md);
}

.modal-concept {
  padding: var(--space-md);
  background: var(--color-bg-tertiary);
  border-radius: var(--radius-md);
  border: 1px solid var(--color-border-primary);
}

.modal-concept-title {
  font-weight: var(--font-weight-semibold);
  margin-bottom: var(--space-xs);
}

.modal-concept-desc {
  color: var(--color-text-secondary);
  font-size: var(--font-size-sm);
  line-height: 1.6;
}

.modal-variables {
  display: flex;
  flex-direction: column;
  gap: var(--space-md);
}

.modal-variable {
  display: flex;
  align-items: center;
  gap: var(--space-md);
  padding: var(--space-md);
  background: var(--color-bg-tertiary);
  border-radius: var(--radius-md);
  border: 1px solid var(--color-border-primary);
}

.modal-variable-icon {
  width: 40px;
  height: 40px;
  border-radius: var(--radius-md);
  background: var(--sim-color-dim, var(--color-accent-primary-dim));
  display: flex;
  align-items: center;
  justify-content: center;
  font-size: var(--font-size-lg);
  flex-shrink: 0;
}

.modal-variable-info {
  flex: 1;
}

.modal-variable-name {
  font-weight: var(--font-weight-medium);
  margin-bottom: 2px;
}

.modal-variable-range {
  font-size: var(--font-size-xs);
  color: var(--color-text-muted);
}

.modal-equations {
  display: flex;
  flex-direction: column;
  gap: var(--space-md);
}

.modal-equation {
  background: var(--color-bg-primary);
  border: 1px solid var(--color-border-primary);
  border-radius: var(--radius-md);
  padding: var(--space-md);
}

.modal-equation-title {
  font-weight: var(--font-weight-semibold);
  color: var(--color-accent-primary);
  margin-bottom: var(--space-sm);
}

.modal-equation-formula {
  font-family: var(--font-family-mono);
  background: var(--color-bg-tertiary);
  padding: var(--space-sm);
  border-radius: var(--radius-sm);
  overflow-x: auto;
  font-size: var(--font-size-sm);
}

.modal-footer {
  display: flex;
  justify-content: flex-end;
  gap: var(--space-md);
  padding: var(--space-xl);
  border-top: 1px solid var(--color-border-primary);
  background: var(--color-bg-tertiary);
}

/* ==========================================================================
   Buttons
   ========================================================================== */
.btn {
  display: inline-flex;
  align-items: center;
  justify-content: center;
  gap: var(--space-sm);
  padding: var(--space-sm) var(--space-lg);
  font-size: var(--font-size-sm);
  font-weight: var(--font-weight-medium);
  border-radius: var(--radius-md);
  transition: all var(--transition-fast);
  white-space: nowrap;
}

.btn:disabled {
  opacity: 0.5;
  cursor: not-allowed;
}

.btn-primary {
  background: var(--color-accent-primary);
  color: var(--color-text-inverse);
}

.btn-primary:hover:not(:disabled) {
  background: #79b8ff;
  transform: translateY(-1px);
  box-shadow: var(--shadow-md);
}

.btn-secondary {
  background: var(--color-bg-tertiary);
  color: var(--color-text-primary);
  border: 1px solid var(--color-border-primary);
}

.btn-secondary:hover:not(:disabled) {
  background: var(--color-bg-hover);
  border-color: var(--color-border-secondary);
}

.btn-danger {
  background: var(--color-accent-danger-dim);
  color: var(--color-accent-danger);
  border: 1px solid var(--color-accent-danger);
}

.btn-danger:hover:not(:disabled) {
  background: var(--color-accent-danger);
  color: var(--color-text-inverse);
}

.btn-icon {
  width: 40px;
  height: 40px;
  padding: 0;
  border-radius: var(--radius-md);
  display: flex;
  align-items: center;
  justify-content: center;
  color: var(--color-text-secondary);
  transition: all var(--transition-fast);
}

.btn-icon:hover:not(:disabled) {
  background: var(--color-bg-tertiary);
  color: var(--color-text-primary);
}

.btn-lg {
  padding: var(--space-md) var(--space-xl);
  font-size: var(--font-size-base);
}

/* ==========================================================================
   Form Elements
   ========================================================================== */
.form-group {
  margin-bottom: var(--space-md);
}

.form-label {
  display: block;
  font-size: var(--font-size-sm);
  font-weight: var(--font-weight-medium);
  margin-bottom: var(--space-xs);
}

.form-input,
.form-select {
  width: 100%;
  padding: var(--space-sm) var(--space-md);
  background: var(--color-bg-tertiary);
  border: 1px solid var(--color-border-primary);
  border-radius: var(--radius-md);
  color: var(--color-text-primary);
  transition: border-color var(--transition-fast), box-shadow var(--transition-fast);
}

.form-input:focus,
.form-select:focus {
  border-color: var(--color-border-focus);
  box-shadow: 0 0 0 3px var(--color-accent-primary-dim);
}

.form-select {
  cursor: pointer;
}

.form-help {
  font-size: var(--font-size-xs);
  color: var(--color-text-muted);
  margin-top: var(--space-xs);
}

/* ==========================================================================
   Error Page
   ========================================================================== */
.error-content {
  display: flex;
  flex-direction: column;
  align-items: center;
  justify-content: center;
  min-height: 60vh;
  text-align: center;
  padding: var(--space-2xl);
}

.error-code {
  font-size: 8rem;
  font-weight: var(--font-weight-bold);
  color: var(--color-accent-danger);
  line-height: 1;
  margin-bottom: var(--space-md);
}

.error-message {
  font-size: var(--font-size-xl);
  color: var(--color-text-secondary);
  margin-bottom: var(--space-xl);
}

/* ==========================================================================
   Footer
   ========================================================================== */
.footer {
  padding: var(--space-lg);
  border-top: 1px solid var(--color-border-primary);
  background: var(--color-bg-secondary);
  margin-top: auto;
}

.footer-content {
  max-width: 1200px;
  margin: 0 auto;
  display: flex;
  align-items: center;
  justify-content: space-between;
  flex-wrap: wrap;
  gap: var(--space-md);
}

.footer-links {
  display: flex;
  gap: var(--space-lg);
}

.footer-links a {
  color: var(--color-text-secondary);
  font-size: var(--font-size-sm);
}

.footer-links a:hover {
  color: var(--color-accent-primary);
}

/* ==========================================================================
   Simulation Canvas UI Elements
   ========================================================================== */
.sim-ui-overlay {
  position: absolute;
  top: var(--space-md);
  right: var(--space-md);
  display: flex;
  flex-direction: column;
  gap: var(--space-sm);
  z-index: 10;
  pointer-events: none;
}

.sim-ui-overlay > * {
  pointer-events: auto;
}

.sim-stats {
  display: flex;
  flex-direction: column;
  gap: var(--space-xs);
  background: rgba(13, 17, 23, 0.9);
  backdrop-filter: blur(10px);
  border: 1px solid var(--color-border-primary);
  border-radius: var(--radius-md);
  padding: var(--space-sm) var(--space-md);
  min-width: 180px;
}

.stat-row {
  display: flex;
  justify-content: space-between;
  gap: var(--space-md);
  font-size: var(--font-size-sm);
}

.stat-label {
  color: var(--color-text-secondary);
}

.stat-value {
  font-family: var(--font-family-mono);
  font-weight: var(--font-weight-medium);
  color: var(--color-accent-primary);
}

/* Trail toggle */
.trail-toggle {
  display: flex;
  align-items: center;
  gap: var(--space-sm);
  padding: var(--space-sm) var(--space-md);
  background: rgba(13, 17, 23, 0.9);
  backdrop-filter: blur(10px);
  border: 1px solid var(--color-border-primary);
  border-radius: var(--radius-full);
  color: var(--color-text-secondary);
  font-size: var(--font-size-sm);
  cursor: pointer;
  transition: all var(--transition-fast);
}

.trail-toggle:hover {
  color: var(--color-text-primary);
  border-color: var(--color-border-secondary);
}

.trail-toggle input {
  width: 16px;
  height: 16px;
  accent-color: var(--color-accent-primary);
}

/* ==========================================================================
   Responsive
   ========================================================================== */
@media (max-width: 1200px) {
  /* Control panel becomes a retractable drawer that slides down from the
     header and floats over the canvas. It is hidden until the header's
     Controls button (or a swipe) sets .open. */
  .control-panel {
    position: fixed;
    top: var(--header-height);
    left: 0;
    right: 0;
    width: 100%;
    max-height: calc(100vh - var(--header-height) - var(--space-lg));
    border-radius: 0 0 var(--radius-lg) var(--radius-lg);
    border-left: none;
    border-right: none;
    overflow-y: auto;
    z-index: var(--z-modal);
    transform: translateY(-100%);
    opacity: 0;
    visibility: hidden;
  }

  .control-panel.open {
    transform: translateY(0);
    opacity: 1;
    visibility: visible;
  }

  /* The desktop collapse mode does not apply to the drawer. */
  .control-panel.collapsed {
    width: 100%;
  }

  .panel-toggle {
    display: none;
  }

  .sim-controls-toggle {
    display: flex;
  }

  /* Highlight the toggle while the drawer is open. */
  body.controls-open .sim-controls-toggle {
    background: var(--color-bg-tertiary);
    color: var(--color-accent-primary);
  }

  .control-panel-backdrop {
    display: block;
  }

  body.controls-open .control-panel-backdrop {
    opacity: 1;
    pointer-events: auto;
  }
}

@media (max-width: 768px) {
  :root {
    --header-height: 56px;
    --panel-width: 100%;
    --info-panel-height: 240px;
  }

  .header {
    padding: 0 var(--space-md);
  }

  .nav {
    display: none;
  }

  .hero {
    padding: calc(var(--header-height) + var(--space-xl)) var(--space-md) var(--space-xl);
  }

  .hero-stats {
    gap: var(--space-lg);
  }

  .simulations-grid {
    grid-template-columns: 1fr;
  }

  .page-header {
    padding: calc(var(--header-height) + var(--space-lg)) var(--space-md) var(--space-lg);
  }

  .page-header-content {
    flex-direction: column;
    align-items: center;
    text-align: center;
  }

  .page-header-icon {
    width: 64px;
    height: 64px;
    font-size: var(--font-size-2xl);
  }

  .modal {
    margin: var(--space-md);
    max-height: calc(100vh - 2 * var(--space-md));
  }

  .modal-header {
    flex-direction: column;
    align-items: center;
    text-align: center;
  }

  .modal-title-group {
    margin-left: 0;
    margin-top: var(--space-md);
  }

  .modal-variable {
    flex-direction: column;
    align-items: flex-start;
  }

  .footer-content {
    flex-direction: column;
    text-align: center;
  }

  .sim-stats {
    min-width: 140px;
    font-size: var(--font-size-xs);
  }
}

@media (max-width: 480px) {
  .hero-actions {
    flex-direction: column;
  }

  .hero-actions .btn {
    width: 100%;
  }

  .sim-dropdown {
    right: -80px;
    min-width: 240px;
  }
}

/* ==========================================================================
   Print Styles
   ========================================================================== */
@media print {
  .header,
  .control-panel,
  .info-panel,
  .btn,
  .modal-overlay {
    display: none !important;
  }

  .simulation-layout {
    height: auto;
  }

  .canvas-container {
    height: 60vh;
    page-break-inside: avoid;
  }

  body {
    background: white;
    color: black;
  }
}

/* ==========================================================================
   Accessibility
   ========================================================================== */
@media (prefers-reduced-motion: reduce) {
  *, *::before, *::after {
    animation-duration: 0.01ms !important;
    animation-iteration-count: 1 !important;
    transition-duration: 0.01ms !important;
  }
}

@media (prefers-contrast: high) {
  :root {
    --color-border-primary: #fff;
    --color-border-secondary: #fff;
    --color-text-secondary: #fff;
    --color-text-muted: #ccc;
  }
}

/* Focus visible only for keyboard navigation */
:focus:not(:focus-visible) {
  outline: none;
}

:focus-visible {
  outline: 2px solid var(--color-border-focus);
  outline-offset: 2px;
}

/* Skip link */
.skip-link {
  position: absolute;
  top: -100%;
  left: 50%;
  transform: translateX(-50%);
  padding: var(--space-sm) var(--space-lg);
  background: var(--color-accent-primary);
  color: var(--color-text-inverse);
  border-radius: var(--radius-md);
  z-index: var(--z-tooltip);
  transition: top var(--transition-fast);
}

.skip-link:focus {
  top: var(--space-md);
}

/* ==========================================================================
   Current-markup compatibility layer
   The page markup (index.php) uses a newer set of class names than the
   original stylesheet (.navbar/.nav-links, .hero-section, .simulation-card,
   .loading-screen, .simulation-view/.sim-header, etc.). These rules style
   those classes using the same design tokens so the menu, home page and
   simulation shell lay out correctly instead of stacking as plain blocks.
   ========================================================================== */

/* ---- Top navigation bar ---- */
.navbar {
  position: fixed;
  top: 0;
  left: 0;
  right: 0;
  height: var(--header-height);
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: var(--space-lg);
  padding: 0 var(--space-lg);
  background: rgba(13, 17, 23, 0.95);
  backdrop-filter: blur(20px);
  border-bottom: 1px solid var(--color-border-primary);
  z-index: var(--z-sticky);
}

.nav-brand {
  display: flex;
  align-items: center;
  gap: var(--space-sm);
  font-size: var(--font-size-lg);
  font-weight: var(--font-weight-bold);
  color: var(--color-text-primary);
  text-decoration: none;
}

.brand-icon {
  font-size: var(--font-size-xl);
  line-height: 1;
}

.nav-links {
  display: flex;
  align-items: center;
  gap: var(--space-xs);
  list-style: none;
  margin: 0;
  padding: 0;
}

.nav-links .nav-link {
  text-decoration: none;
}

/* No JS wiring for the hamburger; keep the links inline at every width. */
.nav-toggle {
  display: none;
}

/* ---- Loading screen ---- */
.loading-screen {
  position: fixed;
  inset: 0;
  z-index: var(--z-modal);
  display: flex;
  flex-direction: column;
  align-items: center;
  justify-content: center;
  gap: var(--space-lg);
  background: var(--color-bg-primary);
  color: var(--color-text-secondary);
  transition: opacity 0.4s ease, visibility 0.4s ease;
}

.loading-spinner {
  width: 48px;
  height: 48px;
  border: 3px solid var(--color-border-primary);
  border-top-color: var(--color-accent-primary);
  border-radius: 50%;
  animation: spin 1s linear infinite;
}

.loading-text {
  font-size: var(--font-size-sm);
  letter-spacing: 0.03em;
}

/* ---- Home: hero ---- */
.hero-section {
  display: flex;
  align-items: center;
  justify-content: center;
  gap: var(--space-2xl);
  flex-wrap: wrap;
  min-height: 100vh;
  padding: calc(var(--header-height) + var(--space-2xl)) var(--space-lg) var(--space-2xl);
  max-width: 1200px;
  margin: 0 auto;
}

.hero-section .hero-content {
  text-align: left;
  flex: 1 1 420px;
}

.hero-title .highlight {
  color: var(--color-accent-primary);
  -webkit-text-fill-color: var(--color-accent-primary);
}

.hero-visual {
  flex: 1 1 360px;
  aspect-ratio: 4 / 3;
  min-width: 280px;
}

.hero-visual canvas {
  width: 100%;
  height: 100%;
  display: block;
}

/* ---- Home: featured simulations + highlights ---- */
.features-section,
.features-highlights {
  max-width: 1200px;
  margin: 0 auto;
  width: 100%;
  padding: var(--space-2xl) var(--space-lg);
}

.features-section .section-title,
.features-highlights .section-title {
  text-align: center;
  margin-bottom: var(--space-xl);
}

.highlights-grid {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(220px, 1fr));
  gap: var(--space-lg);
}

.highlight-card {
  background: var(--color-bg-secondary);
  border: 1px solid var(--color-border-primary);
  border-radius: var(--radius-lg);
  padding: var(--space-xl);
  text-align: center;
}

.highlight-card h3 {
  margin: var(--space-md) 0 var(--space-sm);
  font-size: var(--font-size-lg);
}

.highlight-card p {
  color: var(--color-text-secondary);
  font-size: var(--font-size-sm);
}

.highlight-icon {
  font-size: var(--font-size-3xl);
  line-height: 1;
}

/* ---- Simulation cards (home + list page) ---- */
.simulation-card {
  display: flex;
  flex-direction: column;
  gap: var(--space-sm);
  background: var(--color-bg-secondary);
  border: 1px solid var(--color-border-primary);
  border-radius: var(--radius-lg);
  padding: var(--space-lg);
  transition: transform var(--transition-fast), border-color var(--transition-fast), box-shadow var(--transition-fast);
}

.simulation-card:hover {
  transform: translateY(-3px);
  border-color: var(--color-border-secondary);
  box-shadow: var(--shadow-lg);
}

.card-icon {
  font-size: var(--font-size-3xl);
  line-height: 1;
}

.card-icon.large {
  font-size: var(--font-size-4xl);
}

.card-title {
  font-size: var(--font-size-lg);
  font-weight: var(--font-weight-semibold);
}

.card-description {
  color: var(--color-text-secondary);
  font-size: var(--font-size-sm);
}

.card-concepts {
  display: flex;
  flex-wrap: wrap;
  gap: var(--space-xs);
  margin: var(--space-xs) 0 var(--space-sm);
}

.card-header {
  display: flex;
  align-items: flex-start;
  gap: var(--space-md);
}

.card-variables {
  background: var(--color-bg-tertiary);
  border-radius: var(--radius-md);
  padding: var(--space-md);
  font-size: var(--font-size-sm);
}

.card-variables h4 {
  margin: 0 0 var(--space-sm);
  font-size: var(--font-size-sm);
  color: var(--color-text-secondary);
}

.card-variables ul {
  margin: 0;
  padding-left: var(--space-lg);
  color: var(--color-text-muted);
}

.card-btn {
  margin-top: auto;
  align-self: flex-start;
  text-decoration: none;
}

/* ---- Buttons added by current markup ---- */
.btn-outline {
  background: transparent;
  color: var(--color-text-primary);
  border: 1px solid var(--color-border-secondary);
}

.btn-outline:hover:not(:disabled) {
  background: var(--color-bg-tertiary);
  border-color: var(--color-accent-primary);
  color: var(--color-accent-primary);
}

/* ---- About / list page headers ---- */
.page-header {
  max-width: 1000px;
  margin: 0 auto;
  padding: calc(var(--header-height) + var(--space-2xl)) var(--space-lg) var(--space-lg);
  text-align: center;
}

.page-header h1 {
  font-size: var(--font-size-3xl);
  font-weight: var(--font-weight-bold);
}

.page-subtitle {
  color: var(--color-text-secondary);
  font-size: var(--font-size-md);
  margin-top: var(--space-sm);
}

.about-content,
.simulations-list-page,
.error-content {
  max-width: 1000px;
  margin: 0 auto;
  padding: 0 var(--space-lg) var(--space-2xl);
}

.simulations-list-page {
  max-width: 1200px;
}

.about-section {
  margin-bottom: var(--space-2xl);
}

.about-section h2 {
  font-size: var(--font-size-xl);
  margin-bottom: var(--space-md);
}

.about-section p {
  color: var(--color-text-secondary);
}

.tech-list,
.physics-engines {
  color: var(--color-text-secondary);
  line-height: 1.9;
  padding-left: var(--space-lg);
}

.simulations-list {
  display: flex;
  flex-direction: column;
  gap: var(--space-md);
}

.sim-list-item {
  display: flex;
  gap: var(--space-md);
  background: var(--color-bg-secondary);
  border: 1px solid var(--color-border-primary);
  border-radius: var(--radius-lg);
  padding: var(--space-lg);
}

.sim-list-item .sim-icon {
  font-size: var(--font-size-2xl);
  line-height: 1;
}

.sim-list-item h3 {
  margin-bottom: var(--space-xs);
}

.sim-list-item p {
  color: var(--color-text-secondary);
  font-size: var(--font-size-sm);
}

.sim-concepts {
  display: flex;
  flex-wrap: wrap;
  gap: var(--space-xs);
  margin-top: var(--space-sm);
}

.concept-badge {
  padding: 2px var(--space-sm);
  border-radius: var(--radius-full);
  background: var(--color-bg-tertiary);
  color: var(--color-text-muted);
  font-size: var(--font-size-xs);
}

/* ---- Immersive simulation shell ---- */
/* On the simulation page the site navbar is replaced by the simulation's own
   header bar, which reuses the same --header-height offset the fixed control
   and info panels are positioned against. */
#app[data-page="simulation"] .navbar,
#app[data-page="simulation"] .footer {
  display: none;
}

.simulation-page {
  min-height: 100vh;
}

.simulation-view {
  position: fixed;
  inset: 0;
  flex-direction: column;
  padding-top: var(--header-height);
  padding-bottom: var(--info-panel-height);
}

.sim-header {
  position: fixed;
  top: 0;
  left: 0;
  right: 0;
  height: var(--header-height);
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: var(--space-md);
  padding: 0 var(--space-lg);
  background: rgba(13, 17, 23, 0.95);
  backdrop-filter: blur(20px);
  border-bottom: 1px solid var(--color-border-primary);
  z-index: var(--z-sticky);
}

.sim-header-left,
.sim-header-center,
.sim-header-right {
  display: flex;
  align-items: center;
  gap: var(--space-md);
}

.sim-header-center {
  gap: var(--space-lg);
}

.sim-title {
  display: flex;
  align-items: center;
  gap: var(--space-sm);
}

.sim-title h1 {
  font-size: var(--font-size-md);
  font-weight: var(--font-weight-semibold);
  margin: 0;
}

.sim-title .sim-icon {
  font-size: var(--font-size-lg);
  line-height: 1;
}

.sim-status {
  display: flex;
  align-items: center;
  gap: var(--space-sm);
  font-size: var(--font-size-sm);
  color: var(--color-text-secondary);
}

.status-indicator {
  width: 8px;
  height: 8px;
  border-radius: 50%;
  background: var(--color-text-muted);
}

.status-indicator.running {
  background: var(--color-accent-success);
  box-shadow: 0 0 8px var(--color-accent-success);
}

.status-indicator.paused {
  background: var(--color-accent-warning);
}

.status-indicator.loading {
  background: var(--color-accent-primary);
  animation: spin 1s linear infinite;
}

.sim-time {
  font-family: var(--font-family-mono);
  font-size: var(--font-size-sm);
  color: var(--color-text-secondary);
}

/* Controls drawer toggle + backdrop — only active on small screens. */
.sim-controls-toggle {
  display: none;
}

.control-panel-backdrop {
  display: none;
  position: fixed;
  top: var(--header-height);
  left: 0;
  right: 0;
  bottom: 0;
  background: rgba(1, 4, 9, 0.55);
  z-index: calc(var(--z-modal) - 10);
  opacity: 0;
  pointer-events: none;
  transition: opacity var(--transition-base);
}

/* Live-data rows injected into the control panel */
.data-row {
  display: flex;
  justify-content: space-between;
  gap: var(--space-md);
  padding: var(--space-xs) 0;
  font-size: var(--font-size-sm);
  border-bottom: 1px solid var(--color-border-primary);
}

.data-row:last-child {
  border-bottom: none;
}

.data-empty {
  color: var(--color-text-muted);
  font-size: var(--font-size-sm);
}

/* ---- Compatibility responsive tweaks ---- */
@media (max-width: 768px) {
  .nav-links {
    gap: 2px;
  }

  .nav-links .nav-link {
    padding: var(--space-xs) var(--space-sm);
    font-size: var(--font-size-xs);
  }

  .brand-text {
    display: none;
  }

  .hero-section {
    text-align: center;
  }

  .hero-section .hero-content {
    text-align: center;
  }

  /* Compact simulation top bar: drop the status/timer, tighten spacing and
     let a long title truncate so the action buttons always fit. */
  .sim-header {
    padding: 0 var(--space-sm);
    gap: var(--space-xs);
  }

  .sim-header-center {
    display: none;
  }

  .sim-header-left,
  .sim-header-right {
    gap: 2px;
  }

  .sim-title h1 {
    font-size: var(--font-size-sm);
    max-width: 34vw;
    overflow: hidden;
    text-overflow: ellipsis;
    white-space: nowrap;
  }

  .sim-header .btn-icon {
    width: 38px;
    height: 38px;
  }

  /* Roomier tap targets for sliders on touch screens. */
  .variable-slider {
    height: 26px;
  }
}

@media (max-width: 480px) {
  .sim-title h1 {
    display: none;
  }

  .sim-header .btn-icon {
    width: 34px;
    height: 34px;
  }

  :root {
    --info-panel-height: 210px;
  }

  .hero-actions .btn {
    width: 100%;
  }
}

/* Placed after the base display:none rules so these win the cascade
   (media queries add no specificity — later source order decides). */
@media (max-width: 1200px) {
  .sim-controls-toggle {
    display: flex;
  }

  .control-panel-backdrop {
    display: block;
  }
}