/* ========================================
   FIRST PROOF — Mathematical Manuscript
   A refined dark academic aesthetic
   ======================================== */

@import url('https://fonts.googleapis.com/css2?family=Cormorant+Garamond:ital,wght@0,300;0,400;0,500;0,600;0,700;1,300;1,400;1,500&family=JetBrains+Mono:wght@300;400;500&family=Source+Serif+4:ital,opsz,wght@0,8..60,300;0,8..60,400;0,8..60,600;1,8..60,300;1,8..60,400&display=swap');

:root {
  /* Core palette — ink & parchment in darkness */
  --bg-deep: #0c0b0f;
  --bg-surface: #141318;
  --bg-card: #1a1920;
  --bg-card-hover: #1f1e26;
  --bg-elevated: #24232c;
  --bg-code: #111017;

  /* Text hierarchy */
  --text-primary: #e8e4dc;
  --text-secondary: #a09b8f;
  --text-tertiary: #6b6760;
  --text-muted: #4a4640;

  /* Accent — aged gold for verified/proved */
  --accent-gold: #c9a84c;
  --accent-gold-dim: #8b7535;
  --accent-gold-glow: rgba(201, 168, 76, 0.12);

  /* Status colors */
  --status-proved: #5a9e6f;
  --status-proved-bg: rgba(90, 158, 111, 0.08);
  --status-open: #6b6760;
  --status-open-bg: rgba(107, 103, 96, 0.06);
  --status-lean: #7b93db;
  --status-lean-bg: rgba(123, 147, 219, 0.08);

  /* Field colors — muted academic */
  --field-stochastic: #9b7eb4;
  --field-representation: #7eb4a0;
  --field-combinatorics: #b49b7e;
  --field-spectral: #7e9bb4;
  --field-topology: #b47e8f;
  --field-symplectic: #8fb47e;
  --field-lattice: #b4a07e;
  --field-tensor: #7e8fb4;
  --field-numerical: #b47ea0;

  /* Borders */
  --border-subtle: rgba(232, 228, 220, 0.06);
  --border-medium: rgba(232, 228, 220, 0.10);
  --border-accent: rgba(201, 168, 76, 0.25);

  /* Typography */
  --font-display: 'Cormorant Garamond', 'Georgia', serif;
  --font-body: 'Source Serif 4', 'Georgia', serif;
  --font-mono: 'JetBrains Mono', 'Menlo', monospace;

  /* Spacing */
  --space-xs: 0.25rem;
  --space-sm: 0.5rem;
  --space-md: 1rem;
  --space-lg: 1.5rem;
  --space-xl: 2.5rem;
  --space-2xl: 4rem;

  /* Radius */
  --radius-sm: 4px;
  --radius-md: 8px;
  --radius-lg: 12px;
}

/* ========== RESET & BASE ========== */

*, *::before, *::after {
  margin: 0;
  padding: 0;
  box-sizing: border-box;
}

html {
  font-size: 16px;
  scroll-behavior: smooth;
  -webkit-font-smoothing: antialiased;
  -moz-osx-font-smoothing: grayscale;
}

body {
  font-family: var(--font-body);
  background: var(--bg-deep);
  color: var(--text-primary);
  line-height: 1.7;
  min-height: 100vh;
  overflow-x: hidden;
}

/* Subtle grain texture overlay */
body::before {
  content: '';
  position: fixed;
  top: 0;
  left: 0;
  width: 100%;
  height: 100%;
  pointer-events: none;
  z-index: 9999;
  opacity: 0.025;
  background-image: url("data:image/svg+xml,%3Csvg viewBox='0 0 256 256' xmlns='http://www.w3.org/2000/svg'%3E%3Cfilter id='noiseFilter'%3E%3CfeTurbulence type='fractalNoise' baseFrequency='0.85' numOctaves='4' stitchTiles='stitch'/%3E%3C/filter%3E%3Crect width='100%25' height='100%25' filter='url(%23noiseFilter)'/%3E%3C/svg%3E");
}

/* ========== HEADER / HERO ========== */

.site-header {
  position: relative;
  padding: var(--space-2xl) var(--space-xl);
  border-bottom: 1px solid var(--border-subtle);
  overflow: hidden;
}

.site-header::before {
  content: '';
  position: absolute;
  top: 0;
  left: 0;
  right: 0;
  bottom: 0;
  background: radial-gradient(ellipse at 30% 20%, rgba(201, 168, 76, 0.03) 0%, transparent 60%),
              radial-gradient(ellipse at 70% 80%, rgba(123, 147, 219, 0.02) 0%, transparent 50%);
  pointer-events: none;
}

.header-inner {
  max-width: 1100px;
  margin: 0 auto;
  position: relative;
}

.header-eyebrow {
  font-family: var(--font-mono);
  font-size: 0.7rem;
  font-weight: 400;
  letter-spacing: 0.2em;
  text-transform: uppercase;
  color: var(--accent-gold-dim);
  margin-bottom: var(--space-lg);
  display: flex;
  align-items: center;
  gap: var(--space-sm);
}

.header-eyebrow::before {
  content: '';
  display: inline-block;
  width: 24px;
  height: 1px;
  background: var(--accent-gold-dim);
}

.site-title {
  font-family: var(--font-display);
  font-size: clamp(2.8rem, 6vw, 4.5rem);
  font-weight: 300;
  letter-spacing: -0.02em;
  line-height: 1.1;
  color: var(--text-primary);
  margin-bottom: var(--space-md);
}

.site-title em {
  font-style: italic;
  color: var(--accent-gold);
}

.site-authors {
  font-family: var(--font-body);
  font-size: 1.05rem;
  font-weight: 300;
  color: var(--text-secondary);
  margin-bottom: 0.15rem;
}

.site-date {
  font-family: var(--font-body);
  font-size: 0.88rem;
  font-weight: 300;
  color: var(--text-tertiary);
  margin-bottom: var(--space-sm);
}

.site-subtitle {
  font-family: var(--font-body);
  font-size: 1.05rem;
  font-weight: 300;
  color: var(--text-secondary);
  max-width: 680px;
  line-height: 1.8;
  margin-bottom: var(--space-lg);
}

.header-meta {
  display: flex;
  flex-wrap: wrap;
  gap: var(--space-lg);
  align-items: center;
  font-family: var(--font-mono);
  font-size: 0.72rem;
  color: var(--text-tertiary);
  letter-spacing: 0.03em;
}

.header-meta a {
  color: var(--accent-gold-dim);
  text-decoration: none;
  border-bottom: 1px solid transparent;
  transition: border-color 0.3s, color 0.3s;
}

.header-meta a:hover {
  color: var(--accent-gold);
  border-bottom-color: var(--accent-gold-dim);
}

.meta-divider {
  width: 1px;
  height: 14px;
  background: var(--border-medium);
}

/* ========== STATS BAR ========== */

.stats-bar {
  border-bottom: 1px solid var(--border-subtle);
  padding: var(--space-md) var(--space-xl);
  background: var(--bg-surface);
}

.stats-inner {
  max-width: 1100px;
  margin: 0 auto;
  display: flex;
  gap: var(--space-xl);
  flex-wrap: wrap;
  align-items: center;
}

.stat-item {
  display: flex;
  align-items: baseline;
  gap: var(--space-xs);
}

.stat-number {
  font-family: var(--font-display);
  font-size: 1.6rem;
  font-weight: 600;
  color: var(--text-primary);
}

.stat-label {
  font-family: var(--font-mono);
  font-size: 0.65rem;
  color: var(--text-tertiary);
  text-transform: uppercase;
  letter-spacing: 0.15em;
}

.stat-number.gold { color: var(--accent-gold); }
.stat-number.green { color: var(--status-proved); }
.stat-number.blue { color: var(--status-lean); }

/* ========== METHODOLOGY ========== */

.vision-text {
  font-family: var(--font-body);
  font-size: 1.05rem;
  font-weight: 300;
  font-style: italic;
  color: var(--text-secondary);
  line-height: 1.8;
}

.methodology-section {
  max-width: 1100px;
  margin: 0 auto;
  padding: var(--space-lg) var(--space-xl) 0;
}

.methodology-dropdown {
  border: none;
}

.methodology-toggle {
  cursor: pointer;
  list-style: none;
  display: flex;
  align-items: center;
  gap: 0.5rem;
}

.methodology-toggle::-webkit-details-marker { display: none; }

.methodology-toggle::after {
  content: '\25BC';
  font-size: 0.6rem;
  color: var(--text-tertiary);
  transition: transform 0.3s;
}

details[open] > .methodology-toggle::after {
  transform: rotate(180deg);
}

.methodology-content {
  padding-top: var(--space-md);
}

.methodology-content > p {
  font-family: var(--font-body);
  font-size: 1.05rem;
  font-weight: 300;
  color: var(--text-secondary);
  line-height: 1.8;
  margin-bottom: var(--space-lg);
}

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

.methodology-stage {
  display: flex;
  gap: var(--space-md);
  padding: var(--space-md) var(--space-lg);
  background: var(--bg-surface);
  border: 1px solid var(--border-subtle);
  border-radius: 8px;
}

.stage-number {
  font-family: var(--font-display);
  font-size: 1.6rem;
  font-weight: 300;
  color: var(--accent-gold-dim);
  min-width: 2rem;
  padding-top: 0.1rem;
}

.stage-body h3 {
  font-family: var(--font-display);
  font-size: 1.05rem;
  font-weight: 600;
  color: var(--text-primary);
  margin-bottom: 0.4rem;
}

.stage-body p {
  font-family: var(--font-body);
  font-size: 0.92rem;
  font-weight: 300;
  color: var(--text-secondary);
  line-height: 1.7;
}

.stage-highlight {
  color: var(--accent-gold-dim);
  font-weight: 400;
}

.stage-body a {
  color: var(--accent-gold-dim);
  text-decoration: none;
  border-bottom: 1px solid rgba(139, 117, 53, 0.3);
  transition: color 0.2s, border-color 0.2s;
}

.stage-body a:hover {
  color: var(--accent-gold);
  border-bottom-color: var(--accent-gold);
}

.stage-body code {
  font-family: var(--font-mono);
  font-size: 0.85em;
  color: var(--accent-gold-dim);
  background: var(--bg-elevated);
  padding: 0.1em 0.35em;
  border-radius: 3px;
}

.prompt-dropdown {
  margin-top: 0.75rem;
  border: 1px solid var(--border-subtle);
  border-radius: 6px;
  background: var(--bg-deep);
}

.prompt-toggle {
  font-family: var(--font-body);
  font-size: 0.85rem;
  font-weight: 400;
  color: var(--accent-gold-dim);
  cursor: pointer;
  padding: 0.5rem 0.75rem;
  list-style: none;
  display: flex;
  align-items: center;
  gap: 0.4rem;
}

.prompt-toggle::-webkit-details-marker { display: none; }

.prompt-toggle::after {
  content: '\25BC';
  font-size: 0.5rem;
  color: var(--text-tertiary);
  transition: transform 0.3s;
}

details[open] > .prompt-toggle::after {
  transform: rotate(180deg);
}

.prompt-content {
  padding: 0.75rem 1rem 1rem;
  border-top: 1px solid var(--border-subtle);
  font-family: var(--font-mono);
  font-size: 0.78rem;
  line-height: 1.65;
  color: var(--text-secondary);
  white-space: pre-wrap;
  word-wrap: break-word;
  overflow-x: auto;
  max-height: 500px;
  overflow-y: auto;
}

.prompt-content .prompt-heading {
  font-family: var(--font-body);
  font-weight: 600;
  color: var(--text-primary);
  font-size: 0.85rem;
  display: block;
  margin-top: 0.8rem;
  margin-bottom: 0.2rem;
}

.prompt-content .prompt-heading:first-child {
  margin-top: 0;
}

.prompt-content .prompt-subheading {
  font-family: var(--font-body);
  font-weight: 500;
  color: var(--accent-gold-dim);
  font-size: 0.82rem;
  display: block;
  margin-top: 0.6rem;
  margin-bottom: 0.1rem;
}

.prompt-content .prompt-var {
  color: var(--accent-gold);
  font-style: italic;
}

/* ========== FOOTER ========== */

.site-footer {
  text-align: center;
  padding: var(--space-xl) var(--space-md) var(--space-lg);
  border-top: 1px solid var(--border-subtle);
  margin-top: var(--space-xl);
}

.site-footer p {
  font-family: var(--font-mono);
  font-size: 0.65rem;
  font-weight: 300;
  color: var(--text-muted);
  letter-spacing: 0.03em;
}

.site-footer a {
  color: var(--text-muted);
  text-decoration: none;
  border-bottom: 1px solid rgba(74, 70, 64, 0.4);
  transition: color 0.2s, border-color 0.2s;
}

.site-footer a:hover {
  color: var(--text-tertiary);
  border-bottom-color: var(--text-tertiary);
}

/* ========== PROBLEM LIST ========== */

.problems-section {
  max-width: 1100px;
  margin: 0 auto;
  padding: var(--space-lg) var(--space-xl);
}

.section-title {
  font-family: var(--font-display);
  font-size: 1.4rem;
  font-weight: 400;
  color: var(--text-secondary);
  margin-bottom: var(--space-xl);
  letter-spacing: 0.02em;
}

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

/* ========== PROBLEM CARD ========== */

.problem-card {
  background: var(--bg-card);
  border: 1px solid var(--border-subtle);
  border-radius: var(--radius-md);
  overflow: hidden;
  transition: background 0.3s, border-color 0.3s, box-shadow 0.3s;
  cursor: pointer;
  margin-bottom: var(--space-md);
}

.problem-card:hover {
  background: var(--bg-card-hover);
  border-color: var(--border-medium);
  box-shadow: 0 4px 24px rgba(0,0,0,0.2);
}

.problem-card.active {
  border-color: var(--border-accent);
  box-shadow: 0 0 0 1px var(--border-accent), 0 8px 32px rgba(0,0,0,0.3);
}

.problem-header {
  display: grid;
  grid-template-columns: 56px 1fr auto;
  gap: var(--space-md);
  align-items: center;
  padding: var(--space-lg) var(--space-lg);
  cursor: pointer;
}

.problem-number {
  font-family: var(--font-display);
  font-size: 1.8rem;
  font-weight: 300;
  color: var(--text-muted);
  text-align: center;
  line-height: 1;
}

.problem-card.has-proof .problem-number {
  color: var(--accent-gold-dim);
}

.problem-info {
  min-width: 0;
}

.problem-field {
  font-family: var(--font-mono);
  font-size: 0.65rem;
  font-weight: 500;
  text-transform: uppercase;
  letter-spacing: 0.15em;
  margin-bottom: 4px;
  display: inline-block;
  padding: 2px 8px;
  border-radius: 3px;
}

.problem-title {
  font-family: var(--font-body);
  font-size: 0.95rem;
  font-weight: 400;
  color: var(--text-primary);
  line-height: 1.5;
  display: -webkit-box;
  -webkit-line-clamp: 2;
  -webkit-box-orient: vertical;
  overflow: hidden;
}

.problem-badges {
  display: flex;
  gap: var(--space-sm);
  align-items: center;
  flex-shrink: 0;
}

.badge {
  font-family: var(--font-mono);
  font-size: 0.6rem;
  font-weight: 500;
  letter-spacing: 0.1em;
  text-transform: uppercase;
  padding: 4px 10px;
  border-radius: 3px;
  white-space: nowrap;
}

.badge-proved {
  color: var(--status-proved);
  background: var(--status-proved-bg);
  border: 1px solid rgba(90, 158, 111, 0.15);
}

.badge-lean {
  color: var(--status-lean);
  background: var(--status-lean-bg);
  border: 1px solid rgba(123, 147, 219, 0.15);
}

.badge-open {
  color: var(--status-open);
  background: var(--status-open-bg);
  border: 1px solid rgba(107, 103, 96, 0.1);
}

.expand-icon {
  width: 20px;
  height: 20px;
  color: var(--text-muted);
  transition: transform 0.3s, color 0.3s;
}

.problem-card.active .expand-icon {
  transform: rotate(180deg);
  color: var(--accent-gold-dim);
}

/* ========== PROBLEM EXPANDED CONTENT ========== */

.problem-content {
  display: none;
  border-top: 1px solid var(--border-subtle);
}

.problem-card.active .problem-content {
  display: block;
}

/* Tabs — two-row layout */
.proof-tabs {
  display: flex;
  flex-direction: column;
  border-bottom: 1px solid var(--border-subtle);
  background: rgba(0,0,0,0.15);
}

.tab-row {
  display: flex;
  align-items: center;
  padding: 0 var(--space-lg);
  border-bottom: 1px solid var(--border-subtle);
}

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

.tab-row-label {
  font-family: var(--font-mono);
  font-size: 0.58rem;
  font-weight: 500;
  text-transform: uppercase;
  letter-spacing: 0.15em;
  color: var(--text-muted);
  min-width: 56px;
  padding: var(--space-sm) 0;
  flex-shrink: 0;
}

.tab-row-buttons {
  display: flex;
  flex-wrap: wrap;
}

.proof-tab {
  font-family: var(--font-mono);
  font-size: 0.65rem;
  font-weight: 400;
  letter-spacing: 0.06em;
  text-transform: uppercase;
  padding: var(--space-sm) var(--space-md);
  color: var(--text-tertiary);
  border: none;
  background: none;
  cursor: pointer;
  position: relative;
  transition: color 0.3s;
  white-space: nowrap;
}

.proof-tab:hover {
  color: var(--text-secondary);
}

.proof-tab.active {
  color: var(--accent-gold);
}

.proof-tab.active::after {
  content: '';
  position: absolute;
  bottom: -1px;
  left: var(--space-md);
  right: var(--space-md);
  height: 1px;
  background: var(--accent-gold);
}

.proof-tab.disabled {
  opacity: 0.3;
  cursor: not-allowed;
}

/* Tab panels */
.tab-panel {
  display: none;
  padding: var(--space-xl) var(--space-lg);
  max-height: 70vh;
  overflow-y: auto;
}

.tab-panel.active {
  display: block;
}

/* Custom scrollbar */
.tab-panel::-webkit-scrollbar {
  width: 5px;
}

.tab-panel::-webkit-scrollbar-track {
  background: transparent;
}

.tab-panel::-webkit-scrollbar-thumb {
  background: var(--border-medium);
  border-radius: 3px;
}

.tab-panel::-webkit-scrollbar-thumb:hover {
  background: var(--text-muted);
}

/* ========== MATH CONTENT ========== */

.math-statement {
  font-family: var(--font-body);
  font-size: 1rem;
  line-height: 1.9;
  color: var(--text-primary);
  padding: var(--space-lg);
  background: rgba(201, 168, 76, 0.03);
  border-left: 2px solid var(--accent-gold-dim);
  border-radius: 0 var(--radius-sm) var(--radius-sm) 0;
}

.proof-text {
  font-family: var(--font-body);
  font-size: 0.92rem;
  line-height: 1.85;
  color: var(--text-primary);
}

.proof-text h2 {
  font-family: var(--font-display);
  font-size: 1.5rem;
  font-weight: 500;
  color: var(--text-primary);
  margin: var(--space-xl) 0 var(--space-md);
  padding-bottom: var(--space-sm);
  border-bottom: 1px solid var(--border-subtle);
}

.proof-text h3 {
  font-family: var(--font-display);
  font-size: 1.2rem;
  font-weight: 500;
  color: var(--text-primary);
  margin: var(--space-lg) 0 var(--space-sm);
}

.proof-text h4 {
  font-family: var(--font-body);
  font-size: 1rem;
  font-weight: 600;
  color: var(--text-secondary);
  margin: var(--space-lg) 0 var(--space-sm);
}

.proof-text p {
  margin-bottom: var(--space-md);
}

.proof-text ol, .proof-text ul {
  padding-left: var(--space-lg);
  margin-bottom: var(--space-md);
}

.proof-text li {
  margin-bottom: var(--space-sm);
}

.proof-text strong {
  font-weight: 600;
  color: var(--accent-gold);
}

.proof-text hr {
  border: none;
  border-top: 1px solid var(--border-subtle);
  margin: var(--space-xl) 0;
}

.proof-text code {
  font-family: var(--font-mono);
  font-size: 0.85em;
  background: var(--bg-code);
  padding: 2px 6px;
  border-radius: 3px;
  color: var(--status-lean);
}

.proof-text blockquote {
  border-left: 2px solid var(--accent-gold-dim);
  padding-left: var(--space-lg);
  margin: var(--space-lg) 0;
  color: var(--text-secondary);
  font-style: italic;
}

/* ========== PROOF DOWNLOAD ========== */

.proof-download {
  margin-top: var(--space-lg);
  padding-top: var(--space-md);
  border-top: 1px solid var(--border-subtle);
}

.proof-download a {
  color: var(--text-secondary);
  font-family: var(--font-mono);
  font-size: 0.82em;
  text-decoration: none;
  transition: color var(--transition);
}

.proof-download a:hover {
  color: var(--accent-gold);
}

/* ========== LEAN CODE ========== */

.lean-code-container {
  background: var(--bg-code);
  border: 1px solid var(--border-subtle);
  border-radius: var(--radius-md);
  overflow: hidden;
}

.lean-code-header {
  display: flex;
  justify-content: space-between;
  align-items: center;
  padding: var(--space-sm) var(--space-md);
  background: rgba(123, 147, 219, 0.05);
  border-bottom: 1px solid var(--border-subtle);
}

.lean-code-label {
  font-family: var(--font-mono);
  font-size: 0.65rem;
  font-weight: 500;
  text-transform: uppercase;
  letter-spacing: 0.15em;
  color: var(--status-lean);
}

.lean-code-copy {
  font-family: var(--font-mono);
  font-size: 0.6rem;
  color: var(--text-tertiary);
  background: none;
  border: 1px solid var(--border-subtle);
  padding: 3px 10px;
  border-radius: 3px;
  cursor: pointer;
  transition: color 0.2s, border-color 0.2s;
}

.lean-code-copy:hover {
  color: var(--text-secondary);
  border-color: var(--border-medium);
}

.lean-code {
  padding: var(--space-lg);
  overflow-x: auto;
  font-family: var(--font-mono);
  font-size: 0.8rem;
  line-height: 1.7;
  color: var(--text-primary);
  white-space: pre;
  tab-size: 2;
}

/* Lean syntax highlighting */
.lean-code .keyword { color: #c586c0; }
.lean-code .type { color: #4ec9b0; }
.lean-code .tactic { color: #dcdcaa; }
.lean-code .comment { color: #5a6a4e; font-style: italic; }
.lean-code .string { color: #ce9178; }
.lean-code .operator { color: #d4d4d4; }
.lean-code .namespace { color: #7eb4a0; }
.lean-code .axiom-kw { color: #e06c75; }
.lean-code .theorem-kw { color: #61afef; }
.lean-code .def-kw { color: #c586c0; }
.lean-code .prop { color: #98c379; }

/* ========== EMPTY STATE ========== */

.empty-state {
  text-align: center;
  padding: var(--space-2xl);
  color: var(--text-muted);
}

.empty-state-icon {
  font-size: 2rem;
  margin-bottom: var(--space-md);
  opacity: 0.4;
}

.empty-state-text {
  font-family: var(--font-body);
  font-size: 0.9rem;
  font-style: italic;
}

/* ========== FOOTER ========== */

.site-footer {
  border-top: 1px solid var(--border-subtle);
  padding: var(--space-xl);
  text-align: center;
}

.footer-inner {
  max-width: 1100px;
  margin: 0 auto;
  font-family: var(--font-mono);
  font-size: 0.65rem;
  color: var(--text-muted);
  letter-spacing: 0.05em;
}

.footer-inner a {
  color: var(--text-tertiary);
  text-decoration: none;
  border-bottom: 1px solid transparent;
  transition: color 0.2s, border-color 0.2s;
}

.footer-inner a:hover {
  color: var(--accent-gold-dim);
  border-bottom-color: var(--accent-gold-dim);
}

/* ========== MATHJAX OVERRIDES ========== */

.MathJax {
  font-size: 1em !important;
}

mjx-container[jax="CHTML"][display="true"] {
  margin: var(--space-lg) 0 !important;
  overflow-x: auto;
  overflow-y: hidden;
}

/* ========== ANIMATIONS ========== */

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

.problem-card {
  animation: fadeIn 0.4s ease-out both;
}

.problem-card:nth-child(1) { animation-delay: 0.05s; }
.problem-card:nth-child(2) { animation-delay: 0.08s; }
.problem-card:nth-child(3) { animation-delay: 0.11s; }
.problem-card:nth-child(4) { animation-delay: 0.14s; }
.problem-card:nth-child(5) { animation-delay: 0.17s; }
.problem-card:nth-child(6) { animation-delay: 0.20s; }
.problem-card:nth-child(7) { animation-delay: 0.23s; }
.problem-card:nth-child(8) { animation-delay: 0.26s; }
.problem-card:nth-child(9) { animation-delay: 0.29s; }
.problem-card:nth-child(10) { animation-delay: 0.32s; }

.problem-content {
  animation: fadeIn 0.3s ease-out;
}

/* ========== FIELD COLORS ========== */

.field-stochastic { color: var(--field-stochastic); background: rgba(155, 126, 180, 0.08); }
.field-representation { color: var(--field-representation); background: rgba(126, 180, 160, 0.08); }
.field-combinatorics { color: var(--field-combinatorics); background: rgba(180, 155, 126, 0.08); }
.field-spectral { color: var(--field-spectral); background: rgba(126, 155, 180, 0.08); }
.field-topology { color: var(--field-topology); background: rgba(180, 126, 143, 0.08); }
.field-symplectic { color: var(--field-symplectic); background: rgba(143, 180, 126, 0.08); }
.field-lattice { color: var(--field-lattice); background: rgba(180, 160, 126, 0.08); }
.field-tensor { color: var(--field-tensor); background: rgba(126, 143, 180, 0.08); }
.field-numerical { color: var(--field-numerical); background: rgba(180, 126, 160, 0.08); }

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

@media (max-width: 768px) {
  .site-header {
    padding: var(--space-xl) var(--space-md);
  }

  .problems-section {
    padding: var(--space-xl) var(--space-md);
  }

  .stats-bar {
    padding: var(--space-md);
  }

  .stats-inner {
    gap: var(--space-md);
  }

  .problem-header {
    grid-template-columns: 40px 1fr;
    gap: var(--space-sm);
    padding: var(--space-md);
  }

  .problem-badges {
    grid-column: 1 / -1;
    justify-self: start;
    margin-top: var(--space-xs);
  }

  .problem-number {
    font-size: 1.3rem;
  }

  .proof-tabs {
    padding: 0;
  }

  .tab-row {
    padding: 0 var(--space-sm);
    flex-wrap: wrap;
  }

  .tab-row-label {
    min-width: 44px;
    font-size: 0.52rem;
  }

  .tab-row-buttons {
    overflow-x: auto;
    flex-wrap: nowrap;
  }

  .proof-tab {
    padding: var(--space-xs) var(--space-sm);
    font-size: 0.55rem;
  }

  .tab-panel {
    padding: var(--space-md);
  }

  .math-statement {
    padding: var(--space-md);
    font-size: 0.9rem;
  }

  .lean-code {
    font-size: 0.72rem;
    padding: var(--space-md);
  }

  .header-meta {
    flex-direction: column;
    gap: var(--space-sm);
    align-items: flex-start;
  }

  .meta-divider {
    display: none;
  }
}

@media (max-width: 480px) {
  .site-title {
    font-size: 2rem;
  }

  .stat-number {
    font-size: 1.3rem;
  }
}