/* ═══════════════════════════════════════════
   AP Calculus BC — Design System
   15-Day Mastery · Main Stylesheet
   ═══════════════════════════════════════════ */

/* ── CSS Custom Properties ── */
:root {
  /* Colors */
  --color-primary: #1a56db;
  --color-primary-hover: #1e40af;
  --color-primary-light: #e8effd;
  --color-accent: #f59e0b;
  --color-accent-light: #fef3c7;
  --color-success: #059669;
  --color-success-light: #d1fae5;
  --color-danger: #dc2626;
  --color-danger-light: #fee2e2;

  --color-bg: #ffffff;
  --color-bg-alt: #f8fafc;
  --color-bg-card: #ffffff;
  --color-text: #1e293b;
  --color-text-secondary: #64748b;
  --color-text-muted: #94a3b8;
  --color-border: #e2e8f0;
  --color-border-hover: #cbd5e1;

  /* Typography */
  --font-sans: 'Inter', -apple-system, BlinkMacSystemFont, 'Segoe UI', system-ui, sans-serif;
  --font-mono: 'JetBrains Mono', 'Fira Code', 'Cascadia Code', monospace;
  --font-hand: 'Kalam', 'ZCOOL KuaiLe', cursive;

  --text-xs: 0.75rem;
  --text-sm: 0.875rem;
  --text-base: 1rem;
  --text-lg: 1.125rem;
  --text-xl: 1.25rem;
  --text-2xl: 1.5rem;
  --text-3xl: 1.875rem;
  --text-4xl: 2.25rem;
  --text-5xl: 3rem;

  /* Spacing */
  --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;
  --space-16: 4rem;
  --space-20: 5rem;

  /* Layout */
  --max-width: 1200px;
  --max-width-content: 800px;
  --nav-height: 64px;
  --radius-sm: 6px;
  --radius-md: 10px;
  --radius-lg: 16px;
  --radius-xl: 24px;

  /* Shadows */
  --shadow-sm: 0 1px 2px rgba(0,0,0,0.05);
  --shadow-md: 0 4px 6px -1px rgba(0,0,0,0.07), 0 2px 4px -2px rgba(0,0,0,0.05);
  --shadow-lg: 0 10px 15px -3px rgba(0,0,0,0.08), 0 4px 6px -4px rgba(0,0,0,0.04);
  --shadow-xl: 0 20px 25px -5px rgba(0,0,0,0.1), 0 8px 10px -6px rgba(0,0,0,0.04);

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

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

html {
  font-size: 16px;
  scroll-behavior: smooth;
  -webkit-text-size-adjust: 100%;
}

body {
  font-family: var(--font-sans);
  font-size: var(--text-base);
  line-height: 1.7;
  color: var(--color-text);
  background: var(--color-bg);
  -webkit-font-smoothing: antialiased;
  -moz-osx-font-smoothing: grayscale;
  min-height: 100vh;
  display: flex;
  flex-direction: column;
}

main { flex: 1; }

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

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

/* ── Typography ── */
h1, h2, h3, h4, h5, h6 {
  line-height: 1.3;
  font-weight: 700;
  color: var(--color-text);
}

h1 { font-size: var(--text-4xl); letter-spacing: -0.02em; }
h2 { font-size: var(--text-3xl); letter-spacing: -0.015em; }
h3 { font-size: var(--text-2xl); }
h4 { font-size: var(--text-xl); }

/* ── Utility Classes ── */
.container {
  width: 100%;
  max-width: var(--max-width);
  margin: 0 auto;
  padding: 0 var(--space-4);
}

.container-narrow {
  max-width: var(--max-width-content);
  margin: 0 auto;
  padding: 0 var(--space-4);
}

.sr-only {
  position: absolute; width: 1px; height: 1px;
  padding: 0; margin: -1px; overflow: hidden;
  clip: rect(0,0,0,0); border: 0;
}

.text-center { text-align: center; }
.text-muted { color: var(--color-text-secondary); }
.text-sm { font-size: var(--text-sm); }

/* ── Buttons ── */
.btn {
  display: inline-flex;
  align-items: center;
  justify-content: center;
  gap: var(--space-2);
  padding: 0.625rem 1.5rem;
  font-size: var(--text-base);
  font-weight: 600;
  border-radius: var(--radius-md);
  border: 2px solid transparent;
  cursor: pointer;
  transition: all var(--transition-fast);
  text-decoration: none;
  line-height: 1.5;
  white-space: nowrap;
  font-family: var(--font-sans);
}

.btn:focus-visible {
  outline: 3px solid var(--color-primary);
  outline-offset: 2px;
}

.btn-primary {
  background: var(--color-primary);
  color: #fff;
  border-color: var(--color-primary);
}
.btn-primary:hover {
  background: var(--color-primary-hover);
  border-color: var(--color-primary-hover);
  color: #fff;
}

.btn-accent {
  background: var(--color-accent);
  color: #1e293b;
  border-color: var(--color-accent);
}
.btn-accent:hover {
  filter: brightness(0.95);
  color: #1e293b;
}

.btn-outline {
  background: transparent;
  color: var(--color-primary);
  border-color: var(--color-primary);
}
.btn-outline:hover {
  background: var(--color-primary-light);
  color: var(--color-primary-hover);
}

.btn-lg {
  padding: 0.875rem 2rem;
  font-size: var(--text-lg);
  border-radius: var(--radius-lg);
}

.btn-sm {
  padding: 0.375rem 0.875rem;
  font-size: var(--text-sm);
}

/* ── Navigation ── */
.nav {
  position: sticky;
  top: 0;
  z-index: 1000;
  background: rgba(255,255,255,0.92);
  backdrop-filter: blur(12px);
  -webkit-backdrop-filter: blur(12px);
  border-bottom: 1px solid var(--color-border);
  height: var(--nav-height);
}

.nav-inner {
  display: flex;
  align-items: center;
  justify-content: space-between;
  height: 100%;
}

.nav-brand {
  display: flex;
  align-items: center;
  gap: var(--space-3);
  font-weight: 800;
  font-size: var(--text-xl);
  color: var(--color-text);
  text-decoration: none;
}
.nav-brand span { color: var(--color-primary); }

.nav-links {
  display: flex;
  align-items: center;
  gap: var(--space-1);
}

.nav-links a {
  padding: 0.375rem 0.75rem;
  border-radius: var(--radius-sm);
  color: var(--color-text-secondary);
  font-size: var(--text-sm);
  font-weight: 500;
  transition: all var(--transition-fast);
}
.nav-links a:hover,
.nav-links a.active {
  color: var(--color-primary);
  background: var(--color-primary-light);
}

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

.lang-switch {
  display: flex;
  align-items: center;
  gap: var(--space-1);
  padding: 0.25rem 0.5rem;
  border-radius: var(--radius-sm);
  border: 1px solid var(--color-border);
  background: var(--color-bg);
  cursor: pointer;
  font-size: var(--text-sm);
  color: var(--color-text-secondary);
  transition: all var(--transition-fast);
}
.lang-switch:hover { border-color: var(--color-primary); color: var(--color-primary); }
.lang-switch .active-lang { font-weight: 700; color: var(--color-primary); }

/* Mobile hamburger */
.hamburger {
  display: none;
  flex-direction: column;
  gap: 5px;
  cursor: pointer;
  padding: 4px;
  background: none;
  border: none;
}
.hamburger span {
  display: block;
  width: 24px;
  height: 2px;
  background: var(--color-text);
  border-radius: 2px;
  transition: all var(--transition-fast);
}

@media (max-width: 768px) {
  .hamburger { display: flex; }
  .nav-links {
    display: none;
    position: absolute;
    top: var(--nav-height);
    left: 0; right: 0;
    background: var(--color-bg);
    border-bottom: 1px solid var(--color-border);
    flex-direction: column;
    padding: var(--space-4);
    box-shadow: var(--shadow-lg);
  }
  .nav-links.open { display: flex; }
  .nav-links a { width: 100%; padding: 0.75rem; }
}

/* ── Hero Section ── */
.hero {
  padding: var(--space-20) 0 var(--space-16);
  text-align: center;
  position: relative;
  overflow: hidden;
}

.hero::before {
  content: '';
  position: absolute;
  top: -50%;
  left: -50%;
  width: 200%;
  height: 200%;
  background: radial-gradient(ellipse at 50% 0%, rgba(26,86,219,0.06) 0%, transparent 70%);
  pointer-events: none;
}

.hero-title {
  font-size: clamp(2rem, 5vw, var(--text-5xl));
  font-weight: 900;
  letter-spacing: -0.03em;
  margin-bottom: var(--space-4);
  line-height: 1.15;
}

.hero-title .highlight {
  background: linear-gradient(135deg, var(--color-primary), #7c3aed);
  -webkit-background-clip: text;
  -webkit-text-fill-color: transparent;
  background-clip: text;
}

.hero-subtitle {
  font-size: var(--text-xl);
  color: var(--color-text-secondary);
  max-width: 600px;
  margin: 0 auto var(--space-8);
}

.hero-cta {
  display: flex;
  gap: var(--space-4);
  justify-content: center;
  flex-wrap: wrap;
}

.hero-price {
  margin-top: var(--space-6);
  font-size: var(--text-sm);
  color: var(--color-text-muted);
}
.hero-price strong {
  color: var(--color-accent);
  font-size: var(--text-lg);
}

/* ── Cards ── */
.card {
  background: var(--color-bg-card);
  border: 1px solid var(--color-border);
  border-radius: var(--radius-lg);
  padding: var(--space-6);
  transition: all var(--transition-normal);
}
.card:hover {
  box-shadow: var(--shadow-lg);
  border-color: var(--color-border-hover);
}

/* ── Section ── */
.section {
  padding: var(--space-16) 0;
}

.section-header {
  text-align: center;
  margin-bottom: var(--space-12);
}

.section-header h2 {
  margin-bottom: var(--space-3);
}

.section-header p {
  color: var(--color-text-secondary);
  max-width: 600px;
  margin: 0 auto;
}

/* ── Grids ── */
.grid-2 {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(280px, 1fr));
  gap: var(--space-6);
}

.grid-3 {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(240px, 1fr));
  gap: var(--space-6);
}

.grid-4 {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(200px, 1fr));
  gap: var(--space-4);
}

/* ── Badges & Tags ── */
.badge {
  display: inline-flex;
  align-items: center;
  padding: 0.125rem 0.625rem;
  border-radius: 999px;
  font-size: var(--text-xs);
  font-weight: 600;
}

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

.badge-success {
  background: var(--color-success-light);
  color: var(--color-success);
}

.badge-accent {
  background: var(--color-accent-light);
  color: #92400e;
}

/* ── Footer ── */
.footer {
  background: var(--color-bg-alt);
  border-top: 1px solid var(--color-border);
  padding: var(--space-12) 0 var(--space-8);
  margin-top: auto;
}

.footer-grid {
  display: grid;
  grid-template-columns: 2fr 1fr 1fr;
  gap: var(--space-8);
}

.footer h4 {
  font-size: var(--text-sm);
  text-transform: uppercase;
  letter-spacing: 0.05em;
  margin-bottom: var(--space-4);
  color: var(--color-text-secondary);
}

.footer a {
  display: block;
  color: var(--color-text-muted);
  font-size: var(--text-sm);
  margin-bottom: var(--space-2);
}
.footer a:hover { color: var(--color-text); }

.footer-bottom {
  margin-top: var(--space-8);
  padding-top: var(--space-6);
  border-top: 1px solid var(--color-border);
  text-align: center;
  font-size: var(--text-sm);
  color: var(--color-text-muted);
}

@media (max-width: 768px) {
  .footer-grid { grid-template-columns: 1fr; }
}

/* ── KaTeX Overrides ── */
.katex-display {
  overflow-x: auto;
  overflow-y: hidden;
  padding: var(--space-2) 0;
}

.katex { font-size: 1.1em; }

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

.animate-in {
  animation: fadeInUp 0.5s ease forwards;
}

/* ── Diagnostic / Quiz ── */
.quiz-question {
  background: var(--color-bg-card);
  border: 1px solid var(--color-border);
  border-radius: var(--radius-lg);
  padding: var(--space-6);
  margin-bottom: var(--space-4);
}

.quiz-question .q-number {
  display: inline-flex;
  align-items: center;
  justify-content: center;
  width: 28px; height: 28px;
  border-radius: 50%;
  background: var(--color-primary-light);
  color: var(--color-primary);
  font-weight: 700;
  font-size: var(--text-sm);
  margin-bottom: var(--space-3);
}

.quiz-options {
  display: grid;
  gap: var(--space-2);
  margin-top: var(--space-4);
}

.quiz-option {
  display: flex;
  align-items: center;
  gap: var(--space-3);
  padding: var(--space-3) var(--space-4);
  border: 2px solid var(--color-border);
  border-radius: var(--radius-md);
  cursor: pointer;
  transition: all var(--transition-fast);
  font-size: var(--text-sm);
}
.quiz-option:hover { border-color: var(--color-primary); background: var(--color-primary-light); }
.quiz-option.selected { border-color: var(--color-primary); background: var(--color-primary-light); }
.quiz-option.correct { border-color: var(--color-success); background: var(--color-success-light); }
.quiz-option.wrong { border-color: var(--color-danger); background: var(--color-danger-light); }

.quiz-explanation {
  display: none;
  margin-top: var(--space-4);
  padding: var(--space-4);
  border-radius: var(--radius-md);
  background: #f8fafc;
  border: 1px solid #e2e8f0;
  font-size: var(--text-sm);
  line-height: 1.7;
  color: var(--color-text);
}
.quiz-explanation.correct-exp {
  background: #f0fdf4;
  border-color: #86efac;
}
.quiz-explanation.wrong-exp {
  background: #fef2f2;
  border-color: #fecaca;
}
.ai-tutor-badge {
  display: inline-flex;
  align-items: center;
  gap: 0.35rem;
  font-weight: 700;
  font-size: var(--text-xs);
  color: #1a56db;
  background: #eff6ff;
  padding: 0.25rem 0.6rem;
  border-radius: 999px;
  margin-bottom: var(--space-2);
}

/* ── Course Timeline ── */
.timeline {
  position: relative;
  padding-left: var(--space-8);
}

.timeline::before {
  content: '';
  position: absolute;
  left: 0;
  top: 0;
  bottom: 0;
  width: 3px;
  background: var(--color-border);
  border-radius: 2px;
}

.timeline-item {
  position: relative;
  padding-bottom: var(--space-8);
}

.timeline-item::before {
  content: '';
  position: absolute;
  left: calc(-1 * var(--space-8) + 5px);
  top: 0;
  width: 16px;
  height: 16px;
  border-radius: 50%;
  background: var(--color-primary);
  border: 3px solid var(--color-bg);
  box-shadow: 0 0 0 2px var(--color-primary);
}

.timeline-item.completed::before {
  background: var(--color-success);
  box-shadow: 0 0 0 2px var(--color-success);
}

/* ── Whiteboard ── */
.whiteboard {
  background: linear-gradient(135deg, #fefef9 0%, #f8f5ee 50%, #faf8f2 100%);
  border: 8px solid #d4c9b8;
  border-radius: var(--radius-lg);
  overflow: hidden;
  margin: var(--space-6) 0;
  box-shadow:
    inset 0 1px 3px rgba(0,0,0,0.04),
    0 4px 12px rgba(0,0,0,0.08),
    0 1px 3px rgba(0,0,0,0.06);
  position: relative;
}

/* Simulate whiteboard texture — subtle dust / eraser marks */
.whiteboard::before {
  content: '';
  position: absolute;
  inset: 0;
  background:
    radial-gradient(ellipse at 20% 80%, rgba(200,190,170,0.12) 0%, transparent 50%),
    radial-gradient(ellipse at 75% 20%, rgba(180,175,160,0.08) 0%, transparent 40%),
    radial-gradient(ellipse at 50% 50%, rgba(210,200,180,0.06) 0%, transparent 60%);
  pointer-events: none;
  z-index: 0;
}

.whiteboard-stage {
  min-height: 300px;
  max-height: 520px;
  overflow-y: auto;
  overflow-x: hidden;
  display: block;
  width: 100%;
  padding: var(--space-6) var(--space-8);
  position: relative;
  z-index: 1;
  scroll-behavior: smooth;
}

/* Force each step to take full width so formulas don't bunch in the corner */
.whiteboard-stage .wb-step-row {
  display: block;
  width: 100%;
  margin: 0.4rem 0;
}

/* KaTeX must be left-aligned and full-width inside the whiteboard */
.whiteboard-stage .katex-display,
.whiteboard-stage .katex {
  display: block !important;
  width: 100% !important;
  text-align: left !important;
  margin: 0 !important;
}

.whiteboard-stage .katex-html {
  display: block !important;
  width: 100% !important;
  text-align: left !important;
}

/* Scrollbar styling for whiteboard */
.whiteboard-stage::-webkit-scrollbar { width: 8px; }
.whiteboard-stage::-webkit-scrollbar-track { background: transparent; }
.whiteboard-stage::-webkit-scrollbar-thumb { background: #d4c9b8; border-radius: 4px; }

/* Step rows: full-width, left-aligned */
.wb-step-row {
  display: block;
  width: 100%;
  margin: 0.5rem 0;
  padding: 0.5rem 0;
  animation: wbFadeIn 0.4s ease-out;
  border-bottom: 1px dashed #e8ddd0;
}

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

.wb-step-number {
  display: inline-block;
  width: 28px;
  font-size: 0.8rem;
  color: #a09080;
  font-weight: 600;
  vertical-align: top;
  padding-top: 0.55rem;
}

.wb-step-formula {
  display: inline-block;
  vertical-align: top;
  width: calc(100% - 36px);
  font-size: 1.35rem;
  color: #2d2418;
  line-height: 1.9;
}

/* Older steps fade slightly */
.wb-step-row:not(.wb-step-active) .wb-step-formula {
  color: #8b7d6b;
  font-size: 1.05rem;
}

.wb-step-active {
  background: rgba(26, 86, 219, 0.05);
  border-radius: 6px;
}

.wb-step-active .wb-step-formula {
  color: #1a3d8f;
  font-weight: 700;
  font-size: 1.5rem;
}

/* KaTeX inside whiteboard: left aligned, full width, no centering */
.wb-step-formula .katex-display,
.wb-step-formula .katex {
  display: block !important;
  width: 100% !important;
  text-align: left !important;
  margin: 0 !important;
}

.wb-step-formula .katex-html {
  display: block !important;
  width: 100% !important;
  text-align: left !important;
}

/* Keep handwritten feel but let KaTeX render correctly */
.whiteboard-stage .katex,
.whiteboard-stage .katex * {
  font-family: 'Kalam', 'ZCOOL KuaiLe', cursive !important;
}

/* Formula sheet panel: left-align all math */
#global-formula-sheet .fs-items .katex-display {
  margin: 0.4rem 0 !important;
  overflow-x: auto;
  text-align: left !important;
}

#global-formula-sheet .katex,
#global-formula-sheet .katex-html {
  text-align: left !important;
}

@keyframes wbFadeIn {
  from { opacity: 0; transform: translateY(8px); }
  to { opacity: 1; transform: translateY(0); }
}

/* Force handwritten font inside whiteboard KaTeX elements */
.whiteboard-stage .katex,
.whiteboard-stage .katex * {
  font-family: 'Kalam', 'ZCOOL KuaiLe', cursive !important;
  color: #2d2418 !important;
}

/* Slight marker-pen tilt for a natural handwriting feel */
.whiteboard-stage .katex {
  transform: rotate(-0.5deg);
}

.whiteboard-stage .katex .mord {
  display: inline-block;
  transform: rotate(var(--wb-char-rotate, 0deg));
}

/* Add subtle variation to common symbol positions */
.whiteboard-stage .katex .mord:nth-child(2n) { --wb-char-rotate: -0.8deg; }
.whiteboard-stage .katex .mord:nth-child(3n) { --wb-char-rotate: 0.6deg; }
.whiteboard-stage .katex .mord:nth-child(5n) { --wb-char-rotate: -0.4deg; }
.whiteboard-stage .katex .mord:nth-child(7n) { --wb-char-rotate: 0.5deg; }

.whiteboard-controls {
  display: flex;
  align-items: center;
  justify-content: center;
  gap: var(--space-3);
  padding: var(--space-3) var(--space-4);
  background: rgba(0,0,0,0.03);
  border-top: 1px solid #e2d9c8;
  position: relative;
  z-index: 1;
}

.whiteboard-controls button {
  background: none;
  border: 1px solid #c4b99a;
  border-radius: var(--radius-sm);
  padding: 0.375rem 0.75rem;
  cursor: pointer;
  font-size: var(--text-sm);
  color: #5c4a3a;
  transition: all var(--transition-fast);
}
.whiteboard-controls button:hover { background: rgba(0,0,0,0.06); }
.whiteboard-controls button:disabled { opacity: 0.4; cursor: not-allowed; }

.whiteboard-caption {
  text-align: center;
  padding: var(--space-3) var(--space-4);
  font-size: var(--text-sm);
  color: var(--color-text-secondary);
  border-top: 1px solid #e2d9c8;
  background: rgba(0,0,0,0.02);
  position: relative;
  z-index: 1;
}

/* ── Step Progress ── */
.steps {
  display: flex;
  align-items: center;
  gap: var(--space-2);
  margin: var(--space-6) 0;
}

.step {
  flex: 1;
  height: 4px;
  background: var(--color-border);
  border-radius: 2px;
  transition: background var(--transition-normal);
}

.step.active { background: var(--color-primary); }
.step.completed { background: var(--color-success); }

/* ── Pricing Card ── */
.pricing-card {
  border: 2px solid var(--color-primary);
  border-radius: var(--radius-xl);
  padding: var(--space-8);
  text-align: center;
  max-width: 400px;
  margin: 0 auto;
  position: relative;
}

.pricing-card .price {
  font-size: 3.5rem;
  font-weight: 900;
  color: var(--color-primary);
  line-height: 1;
  margin: var(--space-4) 0;
}

.pricing-card .price small {
  font-size: var(--text-lg);
  font-weight: 500;
  color: var(--color-text-secondary);
}

/* ── FAQ ── */
.faq-item {
  border-bottom: 1px solid var(--color-border);
  padding: var(--space-4) 0;
}

.faq-question {
  display: flex;
  align-items: center;
  justify-content: space-between;
  width: 100%;
  text-align: left;
  background: none;
  border: none;
  padding: var(--space-2) 0;
  cursor: pointer;
  font-size: var(--text-base);
  font-weight: 600;
  color: var(--color-text);
  font-family: var(--font-sans);
}

.faq-answer {
  padding: var(--space-2) 0 var(--space-4);
  color: var(--color-text-secondary);
  font-size: var(--text-sm);
  line-height: 1.7;
  display: none;
}

.faq-answer.open { display: block; }

/* ── Forms ── */
.form-group {
  margin-bottom: var(--space-4);
}

.form-group label {
  display: block;
  font-size: var(--text-sm);
  font-weight: 600;
  margin-bottom: var(--space-2);
  color: var(--color-text);
}

.form-input {
  width: 100%;
  padding: 0.625rem 0.875rem;
  border: 2px solid var(--color-border);
  border-radius: var(--radius-md);
  font-size: var(--text-base);
  font-family: var(--font-sans);
  transition: border-color var(--transition-fast);
  background: var(--color-bg);
  color: var(--color-text);
}
.form-input:focus {
  outline: none;
  border-color: var(--color-primary);
  box-shadow: 0 0 0 3px var(--color-primary-light);
}

.form-error {
  font-size: var(--text-sm);
  color: var(--color-danger);
  margin-top: var(--space-1);
}

/* ── Alerts ── */
.alert {
  padding: var(--space-4);
  border-radius: var(--radius-md);
  font-size: var(--text-sm);
  margin-bottom: var(--space-4);
}

.alert-info { background: var(--color-primary-light); color: var(--color-primary); }
.alert-success { background: var(--color-success-light); color: var(--color-success); }
.alert-warning { background: var(--color-accent-light); color: #92400e; }
.alert-danger { background: var(--color-danger-light); color: var(--color-danger); }

/* ── Scrollbar ── */
::-webkit-scrollbar { width: 8px; height: 8px; }
::-webkit-scrollbar-track { background: var(--color-bg-alt); }
::-webkit-scrollbar-thumb { background: var(--color-border); border-radius: 4px; }
::-webkit-scrollbar-thumb:hover { background: var(--color-text-muted); }

/* ═══════════════════════════════════════════
   Global Formula Sheet
   ═══════════════════════════════════════════ */

#global-formula-sheet {
  position: fixed;
  z-index: 1000;
  pointer-events: none;
}

#global-formula-sheet .fs-trigger {
  position: fixed;
  right: 1rem;
  bottom: 1rem;
  background: var(--color-primary);
  color: #fff;
  border: none;
  border-radius: 999px;
  padding: 0.75rem 1.25rem;
  font-size: 0.95rem;
  font-weight: 700;
  box-shadow: var(--shadow-lg);
  cursor: pointer;
  pointer-events: auto;
  display: flex;
  align-items: center;
  gap: 0.5rem;
  transition: transform 0.2s, box-shadow 0.2s;
}

#global-formula-sheet .fs-trigger:hover {
  transform: translateY(-2px);
  box-shadow: var(--shadow-xl);
}

#global-formula-sheet .fs-trigger span {
  display: inline;
}

@media (max-width: 640px) {
  #global-formula-sheet .fs-trigger span {
    display: none;
  }
}

#global-formula-sheet .fs-overlay {
  position: fixed;
  inset: 0;
  background: rgba(0, 0, 0, 0.35);
  opacity: 0;
  visibility: hidden;
  transition: opacity 0.3s, visibility 0.3s;
  pointer-events: auto;
}

#global-formula-sheet .fs-overlay.open {
  opacity: 1;
  visibility: visible;
}

#global-formula-sheet .fs-panel {
  position: fixed;
  top: 0;
  right: 0;
  width: min(480px, 90vw);
  height: 100%;
  background: #fff;
  box-shadow: -8px 0 30px rgba(0, 0, 0, 0.12);
  transform: translateX(100%);
  transition: transform 0.35s ease;
  display: flex;
  flex-direction: column;
  pointer-events: auto;
}

#global-formula-sheet .fs-panel.open {
  transform: translateX(0);
}

#global-formula-sheet .fs-header {
  display: flex;
  align-items: center;
  justify-content: space-between;
  padding: var(--space-4) var(--space-5);
  border-bottom: 1px solid var(--color-border);
  background: var(--color-bg-alt);
}

#global-formula-sheet .fs-header h3 {
  margin: 0;
  font-size: var(--text-lg);
}

#global-formula-sheet .fs-close {
  background: transparent;
  border: none;
  font-size: 1.25rem;
  color: var(--color-text-secondary);
  cursor: pointer;
  padding: 0.25rem;
  line-height: 1;
}

#global-formula-sheet .fs-close:hover {
  color: var(--color-text);
}

#global-formula-sheet .fs-tabs {
  display: flex;
  gap: var(--space-2);
  padding: var(--space-3) var(--space-4);
  border-bottom: 1px solid var(--color-border);
  overflow-x: auto;
}

#global-formula-sheet .fs-tab {
  flex-shrink: 0;
  padding: 0.5rem 1rem;
  border-radius: 999px;
  border: 1px solid var(--color-border);
  background: #fff;
  color: var(--color-text-secondary);
  font-size: var(--text-sm);
  font-weight: 600;
  cursor: pointer;
  transition: all 0.2s;
}

#global-formula-sheet .fs-tab:hover {
  border-color: var(--color-primary);
  color: var(--color-primary);
}

#global-formula-sheet .fs-tab.active {
  background: var(--color-primary);
  color: #fff;
  border-color: var(--color-primary);
}

#global-formula-sheet .fs-content {
  flex: 1;
  overflow-y: auto;
  padding: var(--space-4) var(--space-5);
  background: #fff;
}

#global-formula-sheet .fs-group {
  margin-bottom: var(--space-5);
}

#global-formula-sheet .fs-group-name {
  font-weight: 700;
  color: var(--color-primary);
  margin-bottom: var(--space-2);
  font-size: var(--text-sm);
  text-transform: uppercase;
  letter-spacing: 0.03em;
}

#global-formula-sheet .fs-items .katex-display {
  margin: 0.4rem 0 !important;
  overflow-x: auto;
}

/* ── Homework System ── */
.hw-header {
  margin-bottom: var(--space-6);
  padding-bottom: var(--space-4);
  border-bottom: 2px solid var(--color-primary-light);
}
.hw-title {
  font-size: var(--text-2xl);
  font-weight: 800;
  color: var(--color-primary);
  margin-bottom: var(--space-2);
}
.hw-subtitle {
  color: var(--color-text-secondary);
  font-size: var(--text-sm);
}
.hw-meta {
  display: flex;
  gap: var(--space-4);
  align-items: center;
  margin-top: var(--space-3);
  font-size: var(--text-sm);
}
.hw-count {
  color: var(--color-text-secondary);
}
.hw-score {
  padding: 0.2rem 0.75rem;
  border-radius: 999px;
  font-weight: 700;
  font-size: var(--text-sm);
}
.hw-score.hw-pass { background: var(--color-success-light); color: var(--color-success); }
.hw-score.hw-fail { background: var(--color-danger-light); color: var(--color-danger); }

.hw-progress-bar {
  height: 6px;
  background: var(--color-border);
  border-radius: 3px;
  margin-bottom: var(--space-6);
  overflow: hidden;
}
.hw-progress-fill {
  height: 100%;
  background: linear-gradient(90deg, var(--color-primary), var(--color-success));
  border-radius: 3px;
  transition: width 0.3s ease;
}

.hw-questions {
  display: flex;
  flex-direction: column;
  gap: var(--space-5);
}

.hw-question {
  padding: var(--space-5);
  border: 1px solid var(--color-border);
  border-radius: var(--radius-lg);
  background: #fff;
  transition: border-color 0.2s;
}
.hw-question:hover {
  border-color: var(--color-primary-light);
}

.hw-q-header {
  display: flex;
  align-items: flex-start;
  gap: var(--space-2);
  margin-bottom: var(--space-4);
  flex-wrap: wrap;
}
.hw-q-number {
  font-weight: 800;
  color: var(--color-primary);
  font-size: var(--text-lg);
  min-width: 2rem;
}
.hw-q-text {
  flex: 1;
  line-height: 1.7;
  font-weight: 500;
}
.hw-topic-badge {
  font-size: var(--text-xs);
  background: var(--color-accent-light);
  color: #92400e;
  padding: 0.15rem 0.5rem;
  border-radius: 999px;
  white-space: nowrap;
}
.hw-bc-badge {
  font-size: var(--text-xs);
  background: var(--color-primary-light);
  color: var(--color-primary);
  padding: 0.15rem 0.5rem;
  border-radius: 999px;
  font-weight: 700;
  white-space: nowrap;
}

.hw-options {
  display: flex;
  flex-direction: column;
  gap: var(--space-2);
}
.hw-option {
  display: flex;
  align-items: flex-start;
  gap: var(--space-3);
  padding: var(--space-3) var(--space-4);
  border: 2px solid var(--color-border);
  border-radius: var(--radius-md);
  cursor: pointer;
  transition: all 0.15s;
}
.hw-option:hover {
  border-color: var(--color-primary);
  background: var(--color-primary-light);
}
.hw-option input[type="radio"] {
  margin-top: 2px;
  accent-color: var(--color-primary);
}
.hw-option-letter {
  font-weight: 700;
  color: var(--color-primary);
  min-width: 1.5rem;
}
.hw-option-text {
  line-height: 1.6;
}
.hw-option.hw-correct {
  border-color: var(--color-success) !important;
  background: var(--color-success-light) !important;
}
.hw-option.hw-wrong {
  border-color: var(--color-danger) !important;
  background: var(--color-danger-light) !important;
}
.hw-option[disabled] {
  cursor: default;
}

.hw-solution {
  margin-top: var(--space-3);
  padding: var(--space-4);
  border-radius: var(--radius-md);
  font-size: var(--text-sm);
  line-height: 1.7;
}
.hw-sol-correct {
  background: var(--color-success-light);
  border: 1px solid var(--color-success);
}
.hw-sol-wrong {
  background: var(--color-danger-light);
  border: 1px solid var(--color-danger);
}
.hw-sol-header {
  font-weight: 700;
  margin-bottom: var(--space-2);
}
.hw-sol-body {
  color: var(--color-text-secondary);
}
.hw-sol-explanation {
  margin-top: var(--space-2);
  padding-top: var(--space-2);
  border-top: 1px dashed var(--color-border);
  color: var(--color-text);
}

.hw-actions {
  display: flex;
  gap: var(--space-3);
  margin: var(--space-6) 0;
  justify-content: center;
  flex-wrap: wrap;
}
.hw-submit-btn:disabled {
  opacity: 0.6;
  cursor: not-allowed;
}

.hw-result-panel {
  margin-top: var(--space-6);
}
.hw-result-card {
  text-align: center;
  padding: var(--space-8);
  border: 2px solid var(--color-primary-light);
  border-radius: var(--radius-xl);
  background: linear-gradient(135deg, #f8fafc, #e8effd);
}
.hw-result-emoji {
  font-size: 3rem;
  margin-bottom: var(--space-2);
}
.hw-result-grade {
  font-size: var(--text-3xl);
  font-weight: 900;
  color: var(--color-primary);
  margin-bottom: var(--space-2);
}
.hw-result-score {
  font-size: var(--text-2xl);
  font-weight: 700;
  color: var(--color-text);
  margin-bottom: var(--space-3);
}
.hw-result-advice {
  font-size: var(--text-base);
  color: var(--color-text-secondary);
  max-width: 400px;
  margin: 0 auto;
}
.hw-wrong-list {
  text-align: left;
  list-style: none;
  padding: 0;
  margin-top: var(--space-4);
}
.hw-wrong-list li {
  padding: var(--space-3);
  margin-bottom: var(--space-2);
  background: #fff;
  border-radius: var(--radius-md);
  border: 1px solid var(--color-border);
  line-height: 1.6;
}

/* ── Homework KaTeX Rendering ── */
.homework-container {
  font-variant-numeric: tabular-nums;
}
.homework-container .katex {
  font-size: 1.1em;
  font-family: 'KaTeX_Math', 'KaTeX_Main', 'Times New Roman', serif;
}
.homework-container .katex-display {
  margin: 0.75rem 0 !important;
  text-align: left !important;
  overflow-x: auto;
  overflow-y: hidden;
}
.homework-container .katex-display .katex {
  font-size: 1.15em;
}
.hw-q-text .katex,
.hw-option-text .katex,
.hw-sol-body .katex,
.hw-sol-explanation .katex,
.hw-wrong-list .katex {
  vertical-align: middle;
  line-height: 1;
}
.hw-option-text .katex {
  font-size: 1.05em;
}
.hw-sol-body .katex,
.hw-sol-explanation .katex {
  font-size: 1.1em;
}
.hw-sol-body {
  line-height: 1.8;
}

/* ═══════════════════════════════════════════
   MathLab — Interactive math visualizations
   ═══════════════════════════════════════════ */
.math-lab {
  background: var(--color-bg-card);
  border: 2px solid var(--color-border);
  border-radius: var(--radius-xl);
  padding: var(--space-5);
  margin: var(--space-6) 0;
  box-shadow: 0 4px 24px rgba(0,0,0,0.06);
}
.math-lab-inner {
  display: grid;
  grid-template-columns: 1fr 1fr;
  gap: var(--space-6);
  align-items: start;
}
@media (max-width: 800px) {
  .math-lab-inner {
    grid-template-columns: 1fr;
  }
}
.math-lab-controls {
  display: flex;
  flex-direction: column;
  gap: var(--space-4);
}
.math-lab-title {
  font-size: var(--text-lg);
  font-weight: 700;
  color: var(--color-text);
}
.math-lab-formula {
  font-size: var(--text-xl);
  color: var(--color-text);
  padding: var(--space-3);
  background: var(--color-bg-alt);
  border-radius: var(--radius-md);
  text-align: center;
}
.math-lab-readout {
  font-size: var(--text-sm);
  color: var(--color-text-secondary);
  padding: var(--space-3);
  background: var(--color-primary-light);
  border-radius: var(--radius-md);
  text-align: left;
  line-height: 1.5;
  min-height: 3rem;
  display: block;
}
.math-lab-sliders {
  display: flex;
  flex-direction: column;
  gap: var(--space-3);
}
.math-lab-slider label {
  display: flex;
  justify-content: space-between;
  font-size: var(--text-sm);
  font-weight: 600;
  color: var(--color-text-secondary);
  margin-bottom: var(--space-1);
}
.math-lab-slider input[type="range"] {
  width: 100%;
  accent-color: var(--color-primary);
  cursor: pointer;
}
.math-lab-note {
  font-size: var(--text-sm);
  color: var(--color-text-secondary);
  line-height: 1.7;
}
.math-lab-graph {
  background: var(--color-bg);
  border: 1px solid var(--color-border);
  border-radius: var(--radius-lg);
  padding: var(--space-2);
  min-height: 280px;
}
.math-lab-graph svg {
  width: 100%;
  height: auto;
  display: block;
}

/* ── f / f' / f'' three-panel linked view ── */
.math-lab-fdf-panel {
  margin-bottom: var(--space-3);
  border: 1px solid var(--color-border);
  border-radius: var(--radius-md);
  padding: var(--space-2);
  background: var(--color-bg);
}
.math-lab-fdf-panel:last-child {
  margin-bottom: 0;
}
.math-lab-fdf-head {
  font-size: var(--text-sm);
  font-weight: 700;
  margin-bottom: var(--space-1);
  text-align: center;
}
.math-lab-fdf-readout {
  font-size: var(--text-sm);
  color: var(--color-text-secondary);
  text-align: center;
  padding-top: var(--space-1);
}

/* ── Print ── */
@media print {
  .nav, .footer, .btn, .whiteboard-controls, .hw-actions { display: none !important; }
  body { font-size: 12pt; }
}