/* ===== Design Tokens ===== */
:root, [data-theme="light"] {
  --font-body: 'Inter', -apple-system, BlinkMacSystemFont, sans-serif;
  --font-mono: 'JetBrains Mono', 'Fira Code', monospace;

  --text-xs:   clamp(0.75rem,  0.7rem  + 0.25vw, 0.875rem);
  --text-sm:   clamp(0.875rem, 0.8rem  + 0.35vw, 1rem);
  --text-base: clamp(1rem,     0.95rem + 0.25vw, 1.125rem);
  --text-lg:   clamp(1.125rem, 1rem    + 0.75vw, 1.5rem);
  --text-xl:   clamp(1.5rem,   1.2rem  + 1.25vw, 2.25rem);

  --space-1:  0.25rem;
  --space-2:  0.5rem;
  --space-3:  0.75rem;
  --space-4:  1rem;
  --space-5:  1.25rem;
  --space-6:  1.5rem;
  --space-8:  2rem;
  --space-10: 2.5rem;
  --space-12: 3rem;

  --radius-sm: 0.375rem;
  --radius-md: 0.5rem;
  --radius-lg: 0.75rem;
  --radius-xl: 1rem;

  --transition-interactive: 180ms cubic-bezier(0.16, 1, 0.3, 1);

  /* Educational theme: deep blue/slate + vibrant accents */
  --color-bg: #f4f6f9;
  --color-surface: #ffffff;
  --color-surface-2: #f8f9fb;
  --color-border: #dde1e8;
  --color-divider: #e8ecf1;

  --color-text: #1a2332;
  --color-text-muted: #5f6d7e;
  --color-text-faint: #9ca8b8;

  --color-primary: #2563eb;
  --color-primary-hover: #1d4ed8;
  --color-primary-light: #dbeafe;

  --color-accent: #059669;
  --color-accent-hover: #047857;
  --color-accent-light: #d1fae5;

  --color-warning: #d97706;
  --color-warning-light: #fef3c7;

  --color-error: #dc2626;
  --color-error-light: #fee2e2;

  --color-tile: #334155;
  --color-tile-text: #ffffff;
  --color-tile-correct: #16a34a;
  --color-tile-wrong: #dc2626;
  --color-tile-empty: transparent;

  --color-g: #2563eb;
  --color-h: #d97706;
  --color-f: #7c3aed;

  --shadow-sm: 0 1px 2px rgba(0,0,0,0.05);
  --shadow-md: 0 4px 12px rgba(0,0,0,0.08);
  --shadow-lg: 0 12px 32px rgba(0,0,0,0.12);
}

[data-theme="dark"] {
  --color-bg: #0f1419;
  --color-surface: #1a2332;
  --color-surface-2: #1f2b3d;
  --color-border: #2d3a4e;
  --color-divider: #253145;

  --color-text: #e2e8f0;
  --color-text-muted: #8899aa;
  --color-text-faint: #5a6a7d;

  --color-primary: #60a5fa;
  --color-primary-hover: #3b82f6;
  --color-primary-light: #1e3a5f;

  --color-accent: #34d399;
  --color-accent-hover: #10b981;
  --color-accent-light: #0d3326;

  --color-warning: #fbbf24;
  --color-warning-light: #3d2e0a;

  --color-error: #f87171;
  --color-error-light: #3b1515;

  --color-tile: #3b82f6;
  --color-tile-text: #ffffff;
  --color-tile-correct: #34d399;
  --color-tile-wrong: #f87171;

  --color-g: #60a5fa;
  --color-h: #fbbf24;
  --color-f: #a78bfa;

  --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 12px 32px rgba(0,0,0,0.5);
}

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

html {
  -webkit-font-smoothing: antialiased;
  -moz-osx-font-smoothing: grayscale;
  text-rendering: optimizeLegibility;
  scroll-behavior: smooth;
  overflow-x: hidden;
}

body {
  min-height: 100dvh;
  line-height: 1.6;
  font-family: var(--font-body);
  font-size: var(--text-base);
  color: var(--color-text);
  background-color: var(--color-bg);
  display: flex;
  flex-direction: column;
  overflow-x: hidden;
}

button { cursor: pointer; background: none; border: none; font: inherit; color: inherit; }
a, button { transition: color var(--transition-interactive), background var(--transition-interactive); }

/* ===== Header ===== */
.header {
  background: var(--color-surface);
  border-bottom: 1px solid var(--color-border);
  padding: var(--space-4) var(--space-6);
  position: sticky;
  top: 0;
  z-index: 100;
  box-shadow: var(--shadow-sm);
}

.header-inner {
  max-width: 1400px;
  margin: 0 auto;
  display: flex;
  align-items: center;
  justify-content: space-between;
}

.logo-area {
  display: flex;
  align-items: center;
  gap: var(--space-3);
}

.logo-icon { color: var(--color-primary); flex-shrink: 0; }

.site-title {
  font-size: var(--text-lg);
  font-weight: 700;
  line-height: 1.2;
}

.site-subtitle {
  font-size: var(--text-xs);
  color: var(--color-text-muted);
  font-weight: 500;
}

[data-theme-toggle] {
  padding: var(--space-2);
  border-radius: var(--radius-md);
  color: var(--color-text-muted);
}
[data-theme-toggle]:hover { background: var(--color-surface-2); color: var(--color-text); }

/* ===== Main Layout ===== */
.main {
  max-width: 1400px;
  margin: 0 auto;
  padding: var(--space-6);
  display: grid;
  grid-template-columns: 1fr 380px;
  gap: var(--space-6);
  flex: 1;
  width: 100%;
}

/* ===== Puzzle Section ===== */
.grids-row {
  display: flex;
  align-items: flex-start;
  gap: var(--space-6);
  justify-content: center;
  flex-wrap: wrap;
}

.grid-container { text-align: center; }

.grid-label {
  font-size: var(--text-sm);
  font-weight: 600;
  text-transform: uppercase;
  letter-spacing: 0.08em;
  color: var(--color-text-muted);
  margin-bottom: var(--space-3);
}

/* ===== Puzzle Grid ===== */
.puzzle-grid {
  display: grid;
  grid-template-columns: repeat(3, 1fr);
  gap: 4px;
  background: var(--color-border);
  border: 3px solid var(--color-border);
  border-radius: var(--radius-lg);
  overflow: hidden;
  width: 210px;
  height: 210px;
}

.tile {
  display: flex;
  align-items: center;
  justify-content: center;
  font-family: var(--font-mono);
  font-size: 1.75rem;
  font-weight: 700;
  background: var(--color-tile);
  color: var(--color-tile-text);
  transition: all 300ms cubic-bezier(0.34, 1.56, 0.64, 1);
  position: relative;
  aspect-ratio: 1;
}

.tile.empty {
  background: var(--color-surface-2);
  color: transparent;
}

.tile.correct {
  background: var(--color-tile-correct);
}

.tile.wrong {
  background: var(--color-tile);
}

.tile.just-moved {
  animation: tilePop 400ms cubic-bezier(0.34, 1.56, 0.64, 1);
  box-shadow: inset 0 0 0 3px rgba(255,255,255,0.4);
}

@keyframes tilePop {
  0% { transform: scale(0.85); }
  50% { transform: scale(1.08); }
  100% { transform: scale(1); }
}

.cost-display {
  margin-top: var(--space-3);
  font-family: var(--font-mono);
  font-size: var(--text-sm);
  padding: var(--space-2) var(--space-4);
  background: var(--color-surface);
  border: 1px solid var(--color-border);
  border-radius: var(--radius-md);
  display: inline-block;
}

.cost-display .cost-g { color: var(--color-g); font-weight: 600; }
.cost-display .cost-h { color: var(--color-h); font-weight: 600; }
.cost-display .cost-f { color: var(--color-f); font-weight: 700; }

.goal-cost {
  color: var(--color-accent);
  font-weight: 600;
}

.arrow-between {
  color: var(--color-text-faint);
  padding-top: 3.5rem;
}

/* ===== Controls ===== */
.controls {
  display: flex;
  gap: var(--space-3);
  justify-content: center;
  margin-top: var(--space-8);
  flex-wrap: wrap;
}

.btn {
  display: inline-flex;
  align-items: center;
  gap: var(--space-2);
  padding: var(--space-3) var(--space-5);
  font-size: var(--text-sm);
  font-weight: 600;
  border-radius: var(--radius-md);
  border: 1px solid transparent;
  transition: all var(--transition-interactive);
}

.btn kbd {
  font-family: var(--font-mono);
  font-size: 0.7rem;
  padding: 2px 6px;
  border-radius: 4px;
  background: rgba(255,255,255,0.2);
  border: 1px solid rgba(255,255,255,0.2);
  opacity: 0.8;
}

.btn-primary {
  background: var(--color-primary);
  color: #fff;
}
.btn-primary:hover { background: var(--color-primary-hover); transform: translateY(-1px); box-shadow: var(--shadow-md); }
.btn-primary:disabled { opacity: 0.5; pointer-events: none; }
.btn-primary kbd { background: rgba(255,255,255,0.15); border-color: rgba(255,255,255,0.15); }

.btn-accent {
  background: var(--color-accent);
  color: #fff;
}
.btn-accent:hover { background: var(--color-accent-hover); transform: translateY(-1px); box-shadow: var(--shadow-md); }
.btn-accent:disabled { opacity: 0.5; pointer-events: none; }
.btn-accent kbd { background: rgba(255,255,255,0.15); border-color: rgba(255,255,255,0.15); }

.btn-secondary {
  background: var(--color-surface);
  color: var(--color-text);
  border-color: var(--color-border);
}
.btn-secondary:hover { background: var(--color-surface-2); transform: translateY(-1px); }
.btn-secondary kbd { background: var(--color-surface-2); border-color: var(--color-border); color: var(--color-text-muted); }

/* ===== Step Counter ===== */
.step-counter {
  text-align: center;
  margin-top: var(--space-4);
  font-size: var(--text-sm);
  font-weight: 600;
  color: var(--color-text-muted);
}

.progress-bar {
  width: 100%;
  max-width: 400px;
  margin: var(--space-2) auto 0;
  height: 4px;
  background: var(--color-divider);
  border-radius: var(--radius-full);
  overflow: hidden;
}

.progress-fill {
  height: 100%;
  background: var(--color-primary);
  border-radius: var(--radius-full);
  transition: width 300ms ease;
  width: 0%;
}

/* ===== Explanation Panel ===== */
.explanation-panel {
  margin-top: var(--space-6);
  background: var(--color-surface);
  border: 1px solid var(--color-border);
  border-radius: var(--radius-lg);
  padding: var(--space-5);
  box-shadow: var(--shadow-sm);
}

.explanation-header {
  display: flex;
  align-items: center;
  gap: var(--space-2);
  font-weight: 700;
  font-size: var(--text-sm);
  color: var(--color-primary);
  margin-bottom: var(--space-3);
  text-transform: uppercase;
  letter-spacing: 0.05em;
}

.explanation-text {
  font-size: var(--text-sm);
  line-height: 1.7;
  color: var(--color-text);
}

.explanation-text strong { color: var(--color-primary); }
.explanation-text .highlight-g { color: var(--color-g); font-weight: 600; font-family: var(--font-mono); }
.explanation-text .highlight-h { color: var(--color-h); font-weight: 600; font-family: var(--font-mono); }
.explanation-text .highlight-f { color: var(--color-f); font-weight: 700; font-family: var(--font-mono); }
.explanation-text .highlight-move { color: var(--color-accent); font-weight: 600; }

/* ===== State Panel (Right Side) ===== */
.state-panel {
  display: flex;
  flex-direction: column;
  gap: var(--space-4);
}

.info-card {
  background: var(--color-surface);
  border: 1px solid var(--color-border);
  border-radius: var(--radius-lg);
  padding: var(--space-4) var(--space-5);
  box-shadow: var(--shadow-sm);
}

.info-card h3 {
  font-size: var(--text-sm);
  font-weight: 700;
  display: flex;
  align-items: center;
  gap: var(--space-2);
  margin-bottom: var(--space-2);
}

.list-badge {
  display: inline-flex;
  align-items: center;
  padding: 2px 8px;
  border-radius: var(--radius-sm);
  font-size: 0.7rem;
  font-weight: 700;
  letter-spacing: 0.08em;
  text-transform: uppercase;
  font-family: var(--font-mono);
}

.open-badge { background: var(--color-primary-light); color: var(--color-primary); }
.closed-badge { background: var(--color-accent-light); color: var(--color-accent); }

.list-count {
  margin-left: auto;
  font-family: var(--font-mono);
  font-size: var(--text-xs);
  color: var(--color-text-muted);
  background: var(--color-surface-2);
  padding: 2px 8px;
  border-radius: var(--radius-full);
}

.list-desc {
  font-size: var(--text-xs);
  color: var(--color-text-muted);
  margin-bottom: var(--space-3);
  line-height: 1.5;
}

.list-content {
  max-height: 180px;
  overflow-y: auto;
  display: flex;
  flex-direction: column;
  gap: 4px;
}

.list-empty {
  font-size: var(--text-xs);
  color: var(--color-text-faint);
  text-align: center;
  padding: var(--space-3);
  font-style: italic;
}

.list-item {
  display: flex;
  align-items: center;
  gap: var(--space-2);
  padding: var(--space-1) var(--space-2);
  background: var(--color-surface-2);
  border-radius: var(--radius-sm);
  font-family: var(--font-mono);
  font-size: 0.72rem;
  line-height: 1.4;
  border: 1px solid transparent;
}

.list-item.active {
  border-color: var(--color-primary);
  background: var(--color-primary-light);
}

.list-item .mini-grid {
  display: grid;
  grid-template-columns: repeat(3, 1fr);
  gap: 1px;
  width: 36px;
  height: 36px;
  flex-shrink: 0;
  border-radius: 3px;
  overflow: hidden;
  background: var(--color-border);
}

.mini-grid .mini-tile {
  display: flex;
  align-items: center;
  justify-content: center;
  font-size: 0.5rem;
  font-weight: 700;
  background: var(--color-tile);
  color: var(--color-tile-text);
}

.mini-grid .mini-tile.mini-empty {
  background: var(--color-surface-2);
  color: transparent;
}

.list-item .item-costs {
  display: flex;
  flex-direction: column;
  gap: 1px;
}

.item-costs span {
  white-space: nowrap;
}

.item-costs .cost-g-val { color: var(--color-g); }
.item-costs .cost-h-val { color: var(--color-h); }
.item-costs .cost-f-val { color: var(--color-f); font-weight: 700; }

/* ===== Formula Card ===== */
.formula-card h3 { margin-bottom: var(--space-3); }

.formula {
  font-family: var(--font-mono);
  font-size: var(--text-lg);
  font-weight: 700;
  text-align: center;
  padding: var(--space-3);
  background: var(--color-surface-2);
  border-radius: var(--radius-md);
  color: var(--color-f);
  margin-bottom: var(--space-3);
}

.formula-legend {
  display: flex;
  flex-direction: column;
  gap: var(--space-1);
  font-size: var(--text-xs);
  color: var(--color-text-muted);
}

.legend-dot {
  display: inline-block;
  width: 10px;
  height: 10px;
  border-radius: 50%;
  margin-right: 4px;
  vertical-align: middle;
}

.g-dot { background: var(--color-g); }
.h-dot { background: var(--color-h); }
.f-dot { background: var(--color-f); }

/* ===== Move History ===== */
.move-history {
  max-height: 180px;
  overflow-y: auto;
  display: flex;
  flex-direction: column;
  gap: 4px;
}

.move-item {
  font-size: var(--text-xs);
  font-family: var(--font-mono);
  padding: var(--space-1) var(--space-2);
  background: var(--color-surface-2);
  border-radius: var(--radius-sm);
  display: flex;
  align-items: center;
  gap: var(--space-2);
}

.move-item .move-num {
  display: inline-flex;
  align-items: center;
  justify-content: center;
  width: 20px;
  height: 20px;
  border-radius: 50%;
  background: var(--color-primary);
  color: #fff;
  font-size: 0.6rem;
  font-weight: 700;
  flex-shrink: 0;
}

.move-item.current { background: var(--color-primary-light); border: 1px solid var(--color-primary); }

/* ===== Footer ===== */
.footer {
  text-align: center;
  padding: var(--space-4) var(--space-6);
  font-size: var(--text-xs);
  color: var(--color-text-faint);
  border-top: 1px solid var(--color-border);
  display: flex;
  flex-direction: column;
  gap: var(--space-1);
}

.footer a {
  color: var(--color-text-muted);
  text-decoration: none;
}
.footer a:hover { color: var(--color-primary); text-decoration: underline; }

/* ===== Scrollbar ===== */
.list-content::-webkit-scrollbar, .move-history::-webkit-scrollbar { width: 4px; }
.list-content::-webkit-scrollbar-track, .move-history::-webkit-scrollbar-track { background: transparent; }
.list-content::-webkit-scrollbar-thumb, .move-history::-webkit-scrollbar-thumb {
  background: var(--color-border);
  border-radius: var(--radius-full);
}

/* Solved celebration */
.puzzle-grid.solved .tile:not(.empty) {
  background: var(--color-tile-correct);
  animation: celebrateTile 600ms cubic-bezier(0.34, 1.56, 0.64, 1) both;
}

@keyframes celebrateTile {
  0% { transform: scale(1); }
  40% { transform: scale(1.1) rotate(2deg); }
  70% { transform: scale(0.95) rotate(-1deg); }
  100% { transform: scale(1) rotate(0); }
}

.puzzle-grid.solved .tile:nth-child(1) { animation-delay: 0ms; }
.puzzle-grid.solved .tile:nth-child(2) { animation-delay: 50ms; }
.puzzle-grid.solved .tile:nth-child(3) { animation-delay: 100ms; }
.puzzle-grid.solved .tile:nth-child(4) { animation-delay: 150ms; }
.puzzle-grid.solved .tile:nth-child(5) { animation-delay: 200ms; }
.puzzle-grid.solved .tile:nth-child(6) { animation-delay: 250ms; }
.puzzle-grid.solved .tile:nth-child(7) { animation-delay: 300ms; }
.puzzle-grid.solved .tile:nth-child(8) { animation-delay: 350ms; }
.puzzle-grid.solved .tile:nth-child(9) { animation-delay: 400ms; }


/* ============================================================
   CONCEPTS SECTION — "Learn the Basics First"
   ============================================================ */

.concepts-section {
  max-width: 1400px;
  margin: 0 auto;
  padding: var(--space-4) var(--space-6) 0;
  width: 100%;
}

/* ===== Toggle Bar ===== */
.concepts-toggle {
  width: 100%;
  display: flex;
  align-items: center;
  gap: var(--space-3);
  padding: var(--space-4) var(--space-5);
  background: var(--color-surface);
  border: 1px solid var(--color-border);
  border-radius: var(--radius-lg);
  box-shadow: var(--shadow-sm);
  cursor: pointer;
  transition: all var(--transition-interactive);
  text-align: left;
}

.concepts-toggle:hover {
  box-shadow: var(--shadow-md);
  border-color: var(--color-primary);
}

.concepts-toggle-icon {
  display: flex;
  align-items: center;
  justify-content: center;
  width: 36px;
  height: 36px;
  border-radius: var(--radius-md);
  background: var(--color-primary-light);
  color: var(--color-primary);
  flex-shrink: 0;
}

.concepts-toggle-text {
  font-size: var(--text-lg);
  font-weight: 700;
  color: var(--color-text);
  flex: 1;
}

.concepts-toggle-hint {
  font-size: var(--text-xs);
  color: var(--color-text-faint);
  display: none;
}

.concepts-section.collapsed .concepts-toggle-hint {
  display: inline;
}

.concepts-chevron {
  color: var(--color-text-muted);
  transition: transform 300ms ease;
  flex-shrink: 0;
}

.concepts-section.collapsed .concepts-chevron {
  transform: rotate(-90deg);
}

/* ===== Concepts Body ===== */
.concepts-body {
  overflow: hidden;
  transition: max-height 500ms cubic-bezier(0.16, 1, 0.3, 1), opacity 300ms ease, padding 300ms ease;
  max-height: 5000px;
  opacity: 1;
  padding-top: var(--space-4);
}

.concepts-section.collapsed .concepts-body {
  max-height: 0;
  opacity: 0;
  padding-top: 0;
}

.concepts-intro {
  font-size: var(--text-sm);
  color: var(--color-text-muted);
  margin-bottom: var(--space-4);
  line-height: 1.6;
}

/* ===== Concept Cards ===== */
.concept-card {
  background: var(--color-surface);
  border: 1px solid var(--color-border);
  border-radius: var(--radius-lg);
  box-shadow: var(--shadow-sm);
  margin-bottom: var(--space-3);
  overflow: hidden;
  transition: box-shadow var(--transition-interactive);
}

.concept-card:hover {
  box-shadow: var(--shadow-md);
}

.concept-header {
  width: 100%;
  display: flex;
  align-items: center;
  gap: var(--space-3);
  padding: var(--space-4) var(--space-5);
  cursor: pointer;
  text-align: left;
  transition: background var(--transition-interactive);
}

.concept-header:hover {
  background: var(--color-surface-2);
}

.concept-num {
  display: inline-flex;
  align-items: center;
  justify-content: center;
  width: 28px;
  height: 28px;
  border-radius: 50%;
  background: var(--color-primary);
  color: #fff;
  font-size: 0.8rem;
  font-weight: 700;
  flex-shrink: 0;
  font-family: var(--font-mono);
}

.concept-title {
  flex: 1;
  font-size: var(--text-base);
  font-weight: 700;
  color: var(--color-text);
}

.concept-chevron {
  color: var(--color-text-muted);
  transition: transform 300ms ease;
  flex-shrink: 0;
}

.concept-card.collapsed .concept-chevron {
  transform: rotate(-90deg);
}

/* ===== Card Content ===== */
.concept-content {
  padding: 0 var(--space-5) var(--space-5);
  font-size: var(--text-sm);
  line-height: 1.7;
  color: var(--color-text);
  overflow: hidden;
  transition: max-height 400ms cubic-bezier(0.16, 1, 0.3, 1), opacity 250ms ease, padding 300ms ease;
  max-height: 2000px;
  opacity: 1;
}

.concept-card.collapsed .concept-content {
  max-height: 0;
  opacity: 0;
  padding-top: 0;
  padding-bottom: 0;
}

.concept-content p {
  margin-bottom: var(--space-3);
}

.concept-content p:last-child {
  margin-bottom: 0;
}

.concept-content strong {
  color: var(--color-text);
}

.concept-content .highlight-g { color: var(--color-g); font-weight: 600; font-family: var(--font-mono); }
.concept-content .highlight-h { color: var(--color-h); font-weight: 600; font-family: var(--font-mono); }
.concept-content .highlight-f { color: var(--color-f); font-weight: 700; font-family: var(--font-mono); }

/* ===== Concept Mini Grids (Card 1) ===== */
.concept-grids-row {
  display: flex;
  align-items: center;
  justify-content: center;
  gap: var(--space-5);
  margin: var(--space-4) 0;
  flex-wrap: wrap;
}

.concept-grid-block {
  text-align: center;
}

.concept-grid-label {
  font-size: var(--text-xs);
  font-weight: 600;
  text-transform: uppercase;
  letter-spacing: 0.08em;
  color: var(--color-text-muted);
  margin-bottom: var(--space-2);
}

.concept-mini-grid {
  display: grid;
  grid-template-columns: repeat(3, 1fr);
  gap: 3px;
  width: 120px;
  height: 120px;
  background: var(--color-border);
  border: 2px solid var(--color-border);
  border-radius: var(--radius-md);
  overflow: hidden;
}

.cm-tile {
  display: flex;
  align-items: center;
  justify-content: center;
  font-family: var(--font-mono);
  font-size: 1.1rem;
  font-weight: 700;
  background: var(--color-tile);
  color: var(--color-tile-text);
}

.cm-tile.cm-empty {
  background: var(--color-surface-2);
  color: transparent;
}

.concept-arrow {
  color: var(--color-text-faint);
  flex-shrink: 0;
}

/* ===== Concept Callout ===== */
.concept-callout {
  background: var(--color-primary-light);
  border-left: 3px solid var(--color-primary);
  padding: var(--space-3) var(--space-4);
  border-radius: 0 var(--radius-md) var(--radius-md) 0;
  font-size: var(--text-sm);
  margin-top: var(--space-3);
  margin-bottom: var(--space-3);
}

/* ===== Analogy Box ===== */
.concept-analogy {
  background: var(--color-warning-light);
  border: 1px solid var(--color-warning);
  border-radius: var(--radius-md);
  padding: var(--space-4);
  margin: var(--space-3) 0;
}

.analogy-label {
  display: inline-block;
  font-size: var(--text-xs);
  font-weight: 700;
  text-transform: uppercase;
  letter-spacing: 0.06em;
  color: var(--color-warning);
  margin-bottom: var(--space-2);
}

.concept-analogy p {
  margin-bottom: 0;
  font-size: var(--text-sm);
  line-height: 1.65;
}

/* ===== Formula Showcase (Card 3) ===== */
.formula-showcase {
  display: flex;
  align-items: center;
  justify-content: center;
  gap: var(--space-3);
  padding: var(--space-5) var(--space-4);
  background: var(--color-surface-2);
  border-radius: var(--radius-lg);
  margin-bottom: var(--space-4);
  font-family: var(--font-mono);
  font-weight: 700;
  flex-wrap: wrap;
}

.formula-showcase-f {
  font-size: 2rem;
  color: var(--color-f);
}
.formula-showcase-eq {
  font-size: 1.5rem;
  color: var(--color-text-muted);
}
.formula-showcase-g {
  font-size: 2rem;
  color: var(--color-g);
}
.formula-showcase-plus {
  font-size: 1.5rem;
  color: var(--color-text-muted);
}
.formula-showcase-h {
  font-size: 2rem;
  color: var(--color-h);
}

/* ===== g/h/f Breakdown (Card 3) ===== */
.ghf-breakdown {
  display: grid;
  grid-template-columns: repeat(3, 1fr);
  gap: var(--space-3);
  margin-bottom: var(--space-4);
}

.ghf-item {
  display: flex;
  flex-direction: column;
  align-items: center;
  text-align: center;
  padding: var(--space-3);
  border-radius: var(--radius-md);
  border: 1px solid var(--color-border);
  background: var(--color-surface);
}

.ghf-symbol {
  font-family: var(--font-mono);
  font-size: 1.2rem;
  font-weight: 700;
  margin-bottom: var(--space-1);
}

.ghf-g .ghf-symbol { color: var(--color-g); }
.ghf-h .ghf-symbol { color: var(--color-h); }
.ghf-f .ghf-symbol { color: var(--color-f); }

.ghf-label {
  font-size: var(--text-xs);
  font-weight: 600;
  color: var(--color-text);
  margin-bottom: var(--space-1);
}

.ghf-analogy {
  font-size: 0.72rem;
  color: var(--color-text-muted);
  font-style: italic;
  line-height: 1.4;
}

/* ===== Example Box (Card 3) ===== */
.concept-example {
  background: var(--color-accent-light);
  border: 1px solid var(--color-accent);
  border-radius: var(--radius-md);
  padding: var(--space-4);
}

.example-label {
  display: inline-block;
  font-size: var(--text-xs);
  font-weight: 700;
  text-transform: uppercase;
  letter-spacing: 0.06em;
  color: var(--color-accent);
  margin-bottom: var(--space-2);
}

.concept-example p {
  margin-bottom: 0;
}

/* ===== Comparison Table (Card 4) ===== */
.comparison-table-wrapper {
  overflow-x: auto;
  margin: var(--space-3) 0;
  border-radius: var(--radius-md);
  border: 1px solid var(--color-border);
}

.comparison-table {
  width: 100%;
  border-collapse: collapse;
  font-size: var(--text-xs);
  font-family: var(--font-mono);
}

.comparison-table th {
  background: var(--color-surface-2);
  padding: var(--space-2) var(--space-3);
  font-weight: 700;
  text-align: left;
  color: var(--color-text-muted);
  text-transform: uppercase;
  letter-spacing: 0.05em;
  font-size: 0.7rem;
  border-bottom: 1px solid var(--color-border);
}

.comparison-table td {
  padding: var(--space-2) var(--space-3);
  border-bottom: 1px solid var(--color-divider);
  color: var(--color-text);
}

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

.comparison-table .match {
  color: var(--color-accent);
  font-weight: 600;
}

.comparison-table .mismatch {
  color: var(--color-error);
  font-weight: 600;
}

.comparison-table .skip {
  color: var(--color-text-faint);
  font-style: italic;
}

/* ===== A* Steps List (Card 5) ===== */
.astar-steps {
  list-style: none;
  counter-reset: astar;
  margin: var(--space-3) 0;
  padding: 0;
}

.astar-steps li {
  counter-increment: astar;
  display: flex;
  align-items: flex-start;
  gap: var(--space-3);
  padding: var(--space-3);
  margin-bottom: var(--space-2);
  background: var(--color-surface-2);
  border-radius: var(--radius-md);
  font-size: var(--text-sm);
  line-height: 1.6;
}

.astar-steps li::before {
  content: counter(astar);
  display: inline-flex;
  align-items: center;
  justify-content: center;
  min-width: 24px;
  height: 24px;
  border-radius: 50%;
  background: var(--color-primary);
  color: #fff;
  font-size: 0.7rem;
  font-weight: 700;
  flex-shrink: 0;
  font-family: var(--font-mono);
  margin-top: 1px;
}

/* ===== Lists Comparison (Card 6) ===== */
.lists-comparison {
  display: grid;
  grid-template-columns: 1fr 1fr;
  gap: var(--space-3);
  margin-bottom: var(--space-3);
}

.list-explain {
  padding: var(--space-4);
  border-radius: var(--radius-md);
  display: flex;
  flex-direction: column;
  gap: var(--space-2);
}

.list-explain strong {
  font-size: var(--text-sm);
  color: var(--color-text);
}

.list-explain p {
  font-size: var(--text-xs);
  color: var(--color-text-muted);
  margin-bottom: 0;
  line-height: 1.5;
}

.open-explain {
  background: var(--color-primary-light);
  border: 1px solid var(--color-primary);
}

.open-explain .list-badge { margin-bottom: var(--space-1); }

.closed-explain {
  background: var(--color-accent-light);
  border: 1px solid var(--color-accent);
}

.closed-explain .list-badge { margin-bottom: var(--space-1); }

/* ===== Start Simulation CTA ===== */
.concepts-cta {
  text-align: center;
  padding: var(--space-6) 0 var(--space-2);
}

.btn-start-sim {
  display: inline-flex;
  align-items: center;
  gap: var(--space-2);
  padding: var(--space-4) var(--space-8);
  font-size: var(--text-base);
  font-weight: 700;
  border-radius: var(--radius-lg);
  background: var(--color-accent);
  color: #fff;
  border: none;
  transition: all var(--transition-interactive);
  box-shadow: var(--shadow-md);
}

.btn-start-sim:hover {
  background: var(--color-accent-hover);
  transform: translateY(-2px);
  box-shadow: var(--shadow-lg);
}

.btn-start-sim kbd {
  font-family: var(--font-mono);
  font-size: 0.7rem;
  padding: 2px 8px;
  border-radius: 4px;
  background: rgba(255,255,255,0.2);
  border: 1px solid rgba(255,255,255,0.2);
  opacity: 0.8;
}

/* ============================================================
   RESPONSIVE DESIGN
   ============================================================ */

/* --- Tablet & Small Desktop (≤ 960px) --- */
@media (max-width: 960px) {
  .main {
    grid-template-columns: 1fr;
    padding: var(--space-4);
  }

  .state-panel {
    order: 2;
  }

  .concepts-section {
    padding: var(--space-3) var(--space-4) 0;
  }
}

/* --- Small Tablets & Large Phones (≤ 700px) --- */
@media (max-width: 700px) {
  .ghf-breakdown {
    grid-template-columns: 1fr;
  }

  .lists-comparison {
    grid-template-columns: 1fr;
  }

  .formula-showcase {
    gap: var(--space-2);
    padding: var(--space-4) var(--space-3);
  }

  .formula-showcase-f,
  .formula-showcase-g,
  .formula-showcase-h {
    font-size: 1.5rem;
  }

  .formula-showcase-eq,
  .formula-showcase-plus {
    font-size: 1.2rem;
  }

  .concept-header {
    padding: var(--space-3) var(--space-4);
  }

  .concept-content {
    padding: 0 var(--space-4) var(--space-4);
  }

  .concept-mini-grid {
    width: 100px;
    height: 100px;
  }

  .cm-tile {
    font-size: 0.9rem;
  }

  .concept-grids-row {
    gap: var(--space-3);
  }

  /* Header goes stacked */
  .header {
    padding: var(--space-3) var(--space-4);
  }

  .header-inner {
    flex-direction: row;
    gap: var(--space-2);
  }

  .site-title {
    font-size: clamp(0.9rem, 3vw, 1.25rem);
  }

  .site-subtitle {
    font-size: clamp(0.65rem, 2vw, 0.8rem);
  }
}

/* --- Mobile (≤ 600px) --- */
@media (max-width: 600px) {
  /* Header stacks on small screens */
  .header-inner {
    flex-direction: column;
    gap: var(--space-2);
    text-align: center;
  }

  .logo-area {
    flex-direction: column;
    gap: var(--space-2);
  }

  .logo-icon {
    width: 28px;
    height: 28px;
  }

  .site-title {
    font-size: 1rem;
    text-align: center;
  }

  .site-subtitle {
    font-size: 0.7rem;
    text-align: center;
  }

  [data-theme-toggle] {
    position: absolute;
    top: var(--space-3);
    right: var(--space-3);
  }

  .header {
    position: relative;
    padding: var(--space-3);
  }

  /* Main layout adjustments */
  .main {
    padding: var(--space-3);
    gap: var(--space-4);
  }

  /* Puzzle grids row — stack vertically */
  .grids-row {
    flex-direction: column;
    align-items: center;
    gap: var(--space-3);
  }

  .puzzle-grid {
    width: min(210px, 55vw);
    height: min(210px, 55vw);
  }

  .tile {
    font-size: clamp(1.1rem, 4vw, 1.75rem);
  }

  .arrow-between {
    padding-top: 0;
    transform: rotate(90deg);
  }

  .arrow-between svg {
    width: 28px;
    height: 28px;
  }

  /* Controls become full-width stacked */
  .controls {
    flex-direction: column;
    align-items: stretch;
    gap: var(--space-2);
    margin-top: var(--space-4);
  }

  .btn {
    justify-content: center;
    padding: var(--space-3) var(--space-4);
    font-size: var(--text-sm);
    min-height: 44px; /* Touch-friendly minimum */
  }

  /* Hide keyboard shortcut badges on mobile */
  .btn kbd {
    display: none;
  }

  .btn-start-sim kbd {
    display: none;
  }

  /* Cost display */
  .cost-display {
    font-size: clamp(0.7rem, 2.5vw, 0.875rem);
    padding: var(--space-2) var(--space-3);
  }

  /* Step counter */
  .step-counter {
    font-size: var(--text-xs);
  }

  .progress-bar {
    max-width: 100%;
  }

  /* Explanation panel */
  .explanation-panel {
    padding: var(--space-3) var(--space-4);
    margin-top: var(--space-4);
  }

  .explanation-text {
    font-size: var(--text-xs);
    line-height: 1.6;
  }

  /* State Panel — cards spacing */
  .state-panel {
    gap: var(--space-3);
  }

  .info-card {
    padding: var(--space-3) var(--space-4);
  }

  .info-card h3 {
    font-size: var(--text-xs);
  }

  .formula {
    font-size: var(--text-base);
    padding: var(--space-2);
  }

  .formula-legend {
    font-size: 0.7rem;
  }

  .list-content {
    max-height: 150px;
  }

  .list-item {
    font-size: 0.65rem;
    padding: var(--space-1);
  }

  .list-item .mini-grid {
    width: 30px;
    height: 30px;
  }

  .mini-grid .mini-tile {
    font-size: 0.4rem;
  }

  .move-history {
    max-height: 150px;
  }

  .move-item {
    font-size: 0.7rem;
  }

  .move-item .move-num {
    width: 18px;
    height: 18px;
    font-size: 0.55rem;
  }

  /* Concepts toggle */
  .concepts-section {
    padding: var(--space-2) var(--space-3) 0;
  }

  .concepts-toggle {
    padding: var(--space-3) var(--space-4);
    gap: var(--space-2);
  }

  .concepts-toggle-text {
    font-size: var(--text-base);
  }

  .concepts-toggle-icon {
    width: 30px;
    height: 30px;
  }

  .concepts-toggle-icon svg {
    width: 16px;
    height: 16px;
  }

  /* Concept cards */
  .concept-card {
    margin-bottom: var(--space-2);
  }

  .concept-num {
    width: 24px;
    height: 24px;
    font-size: 0.7rem;
  }

  .concept-title {
    font-size: var(--text-sm);
  }

  .concept-content {
    font-size: var(--text-xs);
    line-height: 1.6;
    padding: 0 var(--space-3) var(--space-3);
  }

  .concept-callout {
    padding: var(--space-2) var(--space-3);
    font-size: var(--text-xs);
  }

  .concept-analogy {
    padding: var(--space-3);
  }

  .concept-analogy p {
    font-size: var(--text-xs);
  }

  .concept-example {
    padding: var(--space-3);
  }

  .concept-example p {
    font-size: var(--text-xs);
  }

  /* Concept grids need to be smaller */
  .concept-mini-grid {
    width: 90px;
    height: 90px;
  }

  .cm-tile {
    font-size: 0.8rem;
  }

  .concept-arrow svg {
    width: 24px;
    height: 24px;
  }

  /* A* steps list */
  .astar-steps li {
    padding: var(--space-2);
    font-size: var(--text-xs);
    gap: var(--space-2);
  }

  .astar-steps li::before {
    min-width: 20px;
    height: 20px;
    font-size: 0.6rem;
  }

  /* Comparison table */
  .comparison-table th {
    padding: var(--space-1) var(--space-2);
    font-size: 0.6rem;
  }

  .comparison-table td {
    padding: var(--space-1) var(--space-2);
    font-size: 0.7rem;
  }

  /* Start simulation CTA */
  .btn-start-sim {
    width: 100%;
    justify-content: center;
    padding: var(--space-4) var(--space-5);
  }

  .concepts-cta {
    padding: var(--space-4) 0 var(--space-2);
  }

  /* Footer */
  .footer {
    padding: var(--space-3) var(--space-4);
    font-size: 0.7rem;
  }
}

/* --- Very Small Phones (≤ 400px) --- */
@media (max-width: 400px) {
  .main {
    padding: var(--space-2);
  }

  .puzzle-grid {
    width: min(160px, 48vw);
    height: min(160px, 48vw);
  }

  .tile {
    font-size: 1rem;
  }

  .grid-label {
    font-size: 0.7rem;
  }

  .site-title {
    font-size: 0.85rem;
  }

  .site-subtitle {
    font-size: 0.6rem;
  }

  .concept-mini-grid {
    width: 75px;
    height: 75px;
  }

  .cm-tile {
    font-size: 0.7rem;
  }

  .formula-showcase-f,
  .formula-showcase-g,
  .formula-showcase-h {
    font-size: 1.2rem;
  }

  .formula-showcase-eq,
  .formula-showcase-plus {
    font-size: 1rem;
  }

  .concept-title {
    font-size: var(--text-xs);
  }

  .concepts-toggle-text {
    font-size: var(--text-sm);
  }

  .btn {
    font-size: var(--text-xs);
    padding: var(--space-2) var(--space-3);
  }

  .btn-start-sim {
    font-size: var(--text-sm);
    padding: var(--space-3) var(--space-4);
  }

  .explanation-panel {
    padding: var(--space-2) var(--space-3);
  }

  .info-card {
    padding: var(--space-2) var(--space-3);
  }
}

/* --- Touch device optimizations --- */
@media (hover: none) and (pointer: coarse) {
  /* Enlarge tap targets */
  .btn {
    min-height: 48px;
  }

  .concept-header {
    min-height: 48px;
  }

  .concepts-toggle {
    min-height: 48px;
  }

  [data-theme-toggle] {
    min-width: 44px;
    min-height: 44px;
    display: flex;
    align-items: center;
    justify-content: center;
  }

  /* Hide keyboard shortcuts on touch */
  kbd {
    display: none !important;
  }

  /* Prevent double-tap zoom on buttons */
  .btn,
  .concept-header,
  .concepts-toggle,
  [data-theme-toggle] {
    touch-action: manipulation;
  }
}

/* --- Landscape mode on phones --- */
@media (max-height: 500px) and (orientation: landscape) {
  .header {
    padding: var(--space-2) var(--space-4);
    position: relative; /* unstick on landscape for more space */
  }

  .main {
    padding: var(--space-2) var(--space-4);
  }

  .grids-row {
    flex-direction: row;
    gap: var(--space-4);
  }

  .puzzle-grid {
    width: min(160px, 30vh);
    height: min(160px, 30vh);
  }

  .arrow-between {
    transform: none;
    padding-top: 2rem;
  }

  .controls {
    flex-direction: row;
    flex-wrap: wrap;
    justify-content: center;
  }
}