/* ==========================================================
   DESIGN TOKENS
   ========================================================== */
:root {
  --font-display: 'Instrument Serif', Georgia, serif;
  --font-body: 'DM Sans', 'Noto Sans Thai', sans-serif;
  --font-mono: 'JetBrains Mono', 'Fira Code', monospace;
  --font-thai: 'Noto Sans Thai', 'DM Sans', sans-serif;

  --bg-base: #0a0a0f;
  --bg-surface: rgba(17,17,28,0.75);
  --bg-surface-solid: #11111c;
  --bg-elevated: rgba(26,26,42,0.8);
  --bg-card: rgba(20,20,35,0.65);
  --bg-card-hover: rgba(30,30,52,0.8);
  --bg-code: linear-gradient(145deg, #0c0e1a, #141628);

  --text-primary: #eeeef5;
  --text-secondary: #9d9db8;
  --text-tertiary: #5c5c7a;
  --text-inverse: #0a0a0f;

  --border: rgba(255,255,255,0.06);
  --border-subtle: rgba(255,255,255,0.03);
  --border-hover: rgba(255,255,255,0.12);

  --accent: #7c6aff;
  --accent-light: #9d8fff;
  --accent-glow: rgba(124,106,255,0.25);
  --cyan: #22d3ee;
  --cyan-glow: rgba(34,211,238,0.2);
  --gold: #f5c542;
  --emerald: #34d399;
  --rose: #fb7185;
  --orange: #fb923c;

  --radius: 16px;
  --radius-md: 12px;
  --radius-sm: 8px;
  --radius-xs: 6px;
  --radius-full: 9999px;

  --shadow-sm: 0 1px 2px rgba(0,0,0,0.4);
  --shadow: 0 2px 8px rgba(0,0,0,0.4);
  --shadow-md: 0 8px 24px rgba(0,0,0,0.35);
  --shadow-lg: 0 16px 48px rgba(0,0,0,0.4);
  --shadow-glow: 0 0 0 3px var(--accent-glow);

  --ease: cubic-bezier(0.4, 0, 0.2, 1);
  --ease-spring: cubic-bezier(0.34, 1.56, 0.64, 1);
  --transition: 0.25s var(--ease);
  --transition-fast: 0.15s var(--ease);
}

/* ==========================================================
   RESET
   ========================================================== */
*,*::before,*::after{margin:0;padding:0;box-sizing:border-box}
html{scroll-behavior:smooth;-webkit-font-smoothing:antialiased;-moz-osx-font-smoothing:grayscale}
body{
  font-family:var(--font-body);
  background:var(--bg-base);
  color:var(--text-primary);
  line-height:1.65;
  font-size:15px;
  min-height:100vh;
  overflow-x:hidden;
  position:relative;
}

/* Noise texture overlay */
body::before{
  content:'';position:fixed;inset:0;z-index:0;pointer-events:none;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='n'%3E%3CfeTurbulence type='fractalNoise' baseFrequency='0.9' numOctaves='4' stitchTiles='stitch'/%3E%3C/filter%3E%3Crect width='100%25' height='100%25' filter='url(%23n)'/%3E%3C/svg%3E");
  background-repeat:repeat;
}

/* Ambient glow orbs */
.ambient-orbs{position:fixed;inset:0;z-index:0;pointer-events:none;overflow:hidden}
.orb{position:absolute;border-radius:50%;filter:blur(120px);opacity:0.12;animation:orbFloat 20s ease-in-out infinite}
.orb-1{width:600px;height:600px;background:#7c6aff;top:-10%;left:-5%;animation-delay:0s}
.orb-2{width:500px;height:500px;background:#22d3ee;top:40%;right:-10%;animation-delay:-7s;animation-duration:25s}
.orb-3{width:450px;height:450px;background:#fb7185;bottom:-5%;left:30%;animation-delay:-14s;animation-duration:22s}
@keyframes orbFloat{
  0%,100%{transform:translate(0,0) scale(1)}
  33%{transform:translate(40px,-30px) scale(1.05)}
  66%{transform:translate(-20px,40px) scale(0.95)}
}

/* Scrollbar */
::-webkit-scrollbar{width:5px;height:5px}
::-webkit-scrollbar-track{background:transparent}
::-webkit-scrollbar-thumb{background:rgba(255,255,255,0.1);border-radius:3px}
::-webkit-scrollbar-thumb:hover{background:rgba(255,255,255,0.2)}
::selection{background:var(--accent);color:#fff}

/* Page wrapper */
.page-wrap{position:relative;z-index:1}

/* ==========================================================
   TOPBAR
   ========================================================== */
.topbar{
  position:sticky;top:0;z-index:100;
  background:rgba(10,10,15,0.7);
  backdrop-filter:blur(24px) saturate(180%);
  -webkit-backdrop-filter:blur(24px) saturate(180%);
  border-bottom:1px solid var(--border);
  padding:18px 32px;
}
.topbar-inner{max-width:1400px;margin:0 auto;display:flex;justify-content:space-between;align-items:center;flex-wrap:wrap;gap:12px}
.topbar-brand h1{
  font-family:var(--font-display);font-size:1.85em;font-weight:400;font-style:normal;
  background:linear-gradient(135deg,#7c6aff 0%,#22d3ee 50%,#7c6aff 100%);
  background-size:200% auto;
  -webkit-background-clip:text;-webkit-text-fill-color:transparent;background-clip:text;
  animation:shimmer 4s linear infinite;
  letter-spacing:-0.01em;
}
@keyframes shimmer{0%{background-position:0% center}100%{background-position:200% center}}
.topbar-brand p{font-size:0.8em;color:var(--text-tertiary);margin-top:2px;font-family:var(--font-thai)}
.topbar-actions{display:flex;gap:8px;flex-wrap:wrap}
.topbar-btn{
  display:inline-flex;align-items:center;gap:6px;
  background:rgba(255,255,255,0.04);border:1px solid var(--border);
  color:var(--text-secondary);padding:8px 18px;border-radius:var(--radius-full);
  cursor:pointer;font-size:0.82em;font-weight:600;font-family:var(--font-body);
  transition:var(--transition);white-space:nowrap;
}
.topbar-btn:hover{background:rgba(255,255,255,0.08);color:var(--text-primary);border-color:var(--border-hover);box-shadow:0 0 20px rgba(124,106,255,0.08)}
.topbar-btn.active{background:var(--accent);border-color:var(--accent);color:#fff;box-shadow:0 0 24px var(--accent-glow)}
.topbar-btn svg{width:14px;height:14px}

/* ==========================================================
   CONTROLS BAR
   ========================================================== */
.controls-wrap{
  position:sticky;top:68px;z-index:99;
  background:rgba(10,10,15,0.65);
  backdrop-filter:blur(20px) saturate(180%);
  -webkit-backdrop-filter:blur(20px) saturate(180%);
  border-bottom:1px solid var(--border);
  padding:14px 32px;
}
.controls{max-width:1400px;margin:0 auto;display:flex;flex-wrap:wrap;gap:10px;align-items:center}
.search-wrap{flex:1;min-width:220px;position:relative}
.search-icon{position:absolute;left:14px;top:50%;transform:translateY(-50%);color:var(--text-tertiary);pointer-events:none}
.search-icon svg{width:16px;height:16px}
.search-input{
  width:100%;padding:10px 16px 10px 42px;
  background:rgba(255,255,255,0.04);border:1.5px solid var(--border);border-radius:var(--radius-md);
  font-size:0.9em;font-family:var(--font-thai);font-weight:500;
  color:var(--text-primary);outline:none;transition:var(--transition);
}
.search-input::placeholder{color:var(--text-tertiary);font-weight:400}
.search-input:focus{border-color:var(--accent);box-shadow:var(--shadow-glow);background:rgba(255,255,255,0.06)}

.filter-select{
  padding:10px 14px;background:rgba(255,255,255,0.04);border:1.5px solid var(--border);
  border-radius:var(--radius-md);font-size:0.85em;font-family:var(--font-thai);font-weight:600;
  color:var(--text-primary);cursor:pointer;outline:none;transition:var(--transition);
  -webkit-appearance:none;appearance:none;
  background-image:url("data:image/svg+xml,%3Csvg width='10' height='6' viewBox='0 0 10 6' fill='none' xmlns='http://www.w3.org/2000/svg'%3E%3Cpath d='M1 1L5 5L9 1' stroke='%239d9db8' stroke-width='1.5' stroke-linecap='round'/%3E%3C/svg%3E");
  background-repeat:no-repeat;background-position:right 12px center;padding-right:32px;
}
.filter-select:focus{border-color:var(--accent);box-shadow:var(--shadow-glow)}
.filter-select option{background:var(--bg-base);color:var(--text-primary)}

.view-pills{display:flex;gap:2px;background:rgba(255,255,255,0.03);padding:3px;border-radius:var(--radius-md);border:1px solid var(--border-subtle)}
.view-pill{
  padding:7px 16px;border:none;border-radius:var(--radius-sm);
  background:transparent;color:var(--text-tertiary);cursor:pointer;
  font-size:0.82em;font-weight:600;font-family:var(--font-thai);
  transition:var(--transition);white-space:nowrap;
}
.view-pill:hover{color:var(--text-secondary);background:rgba(255,255,255,0.04)}
.view-pill.active{background:var(--accent);color:#fff;box-shadow:0 0 16px var(--accent-glow)}

.stats-chip{font-size:0.78em;font-weight:600;color:var(--text-tertiary);padding:6px 14px;background:rgba(255,255,255,0.03);border-radius:var(--radius-full);border:1px solid var(--border-subtle);white-space:nowrap}
.kbd-hint{font-size:0.72em;color:var(--text-tertiary);display:flex;gap:6px;align-items:center}
.kbd{display:inline-flex;align-items:center;justify-content:center;min-width:22px;height:20px;padding:0 5px;background:rgba(255,255,255,0.06);border:1px solid var(--border);border-radius:4px;font-family:var(--font-mono);font-size:0.85em;font-weight:500;color:var(--text-secondary)}

/* ==========================================================
   SUBJECT NAV
   ========================================================== */
.subject-nav-wrap{
  background:rgba(10,10,15,0.5);border-bottom:1px solid var(--border-subtle);
  padding:12px 32px;overflow-x:auto;
}
.subject-nav-wrap::-webkit-scrollbar{height:0}
.subject-nav{max-width:1400px;margin:0 auto;display:flex;gap:6px;flex-wrap:nowrap}
.subject-chip{
  display:inline-flex;align-items:center;gap:6px;
  padding:7px 14px;border:1.5px solid var(--border);border-radius:var(--radius-full);
  background:rgba(255,255,255,0.02);cursor:pointer;
  font-size:0.78em;font-weight:600;font-family:var(--font-thai);color:var(--text-secondary);
  transition:var(--transition);white-space:nowrap;flex-shrink:0;
}
.subject-chip:hover{border-color:var(--accent-light);color:var(--accent-light);transform:translateY(-1px);background:rgba(124,106,255,0.06)}
.subject-chip.active{color:#fff;border-color:transparent;box-shadow:0 0 20px rgba(0,0,0,0.3)}
.subject-chip .color-dot{width:8px;height:8px;border-radius:50%;flex-shrink:0}
.chip-badge{font-size:0.8em;background:rgba(255,255,255,0.06);padding:1px 7px;border-radius:var(--radius-full);color:var(--text-tertiary);font-weight:700}
.subject-chip.active .chip-badge{background:rgba(255,255,255,0.2);color:#fff}

/* ==========================================================
   CONTENT
   ========================================================== */
.content{max-width:1400px;margin:0 auto;padding:28px 32px 100px}

/* ==========================================================
   SUBJECT SECTION
   ========================================================== */
.subject-section{margin-bottom:36px}

.subject-header{
  padding:18px 24px;border-radius:var(--radius) var(--radius) 0 0;
  cursor:pointer;display:flex;justify-content:space-between;align-items:center;
  user-select:none;transition:var(--transition);position:relative;overflow:hidden;
}
.subject-header::after{
  content:'';position:absolute;inset:0;
  background:linear-gradient(180deg,rgba(255,255,255,0.05) 0%,transparent 100%);
  pointer-events:none;
}
.subject-header:hover{filter:brightness(1.08)}
.subject-header h2{font-family:var(--font-body);font-size:1em;font-weight:700;color:#fff;position:relative;z-index:1;letter-spacing:-0.01em}
.subject-header-right{display:flex;align-items:center;gap:10px;position:relative;z-index:1}
.subject-count{background:rgba(255,255,255,0.15);padding:3px 12px;border-radius:var(--radius-full);font-size:0.78em;color:rgba(255,255,255,0.9);font-weight:600;backdrop-filter:blur(4px)}
.subject-arrow{font-size:0.85em;color:rgba(255,255,255,0.6);transition:transform 0.3s var(--ease)}
.subject-header.collapsed .subject-arrow{transform:rotate(-90deg)}

.subject-body{
  border:1px solid var(--border);border-top:none;
  border-radius:0 0 var(--radius) var(--radius);
  background:var(--bg-card);
  backdrop-filter:blur(8px);
  overflow:hidden;
}
.subject-body.hidden{display:none}

/* Progress bar */
.progress-track{height:2px;background:rgba(255,255,255,0.03)}
.progress-fill{height:100%;background:linear-gradient(90deg,var(--accent),var(--emerald));transition:width 0.6s var(--ease);border-radius:0 2px 2px 0}

/* ==========================================================
   CATEGORY
   ========================================================== */
.category-header{
  padding:12px 24px;background:rgba(255,255,255,0.02);border-bottom:1px solid var(--border-subtle);
  cursor:pointer;display:flex;justify-content:space-between;align-items:center;transition:var(--transition);
}
.category-header:hover{background:rgba(124,106,255,0.05)}
.category-title{font-weight:700;font-size:0.86em;color:var(--text-primary);display:flex;align-items:center;gap:8px;font-family:var(--font-thai)}
.category-count{font-size:0.75em;color:var(--text-tertiary);font-weight:600}
.category-arrow{font-size:0.7em;color:var(--text-tertiary);transition:transform 0.2s var(--ease)}
.category-header.collapsed .category-arrow{transform:rotate(-90deg)}
.category-body.hidden{display:none}

/* ==========================================================
   FORMULA CARD
   ========================================================== */
.formula-card{
  padding:20px 24px;border-bottom:1px solid var(--border-subtle);
  transition:var(--transition);position:relative;
  background:transparent;
}
.formula-card::before{
  content:'';position:absolute;top:0;left:0;width:3px;height:100%;
  border-radius:0 2px 2px 0;opacity:0;transition:opacity 0.3s var(--ease);
}
.formula-card:hover{background:var(--bg-card-hover)}
.formula-card:hover::before{opacity:1}
.formula-card:last-child{border-bottom:none}
.formula-card.highlighted{background:rgba(124,106,255,0.08);box-shadow:inset 3px 0 0 var(--accent)}

.formula-top{display:flex;justify-content:space-between;align-items:flex-start;gap:12px}
.formula-name{display:flex;align-items:center;gap:6px;flex-wrap:wrap;margin-bottom:3px}
.formula-name-text{font-weight:700;font-size:0.95em;color:var(--text-primary);letter-spacing:-0.01em}

.tag{
  display:inline-flex;align-items:center;
  font-size:0.6em;font-weight:700;text-transform:uppercase;letter-spacing:0.08em;
  padding:3px 8px;border-radius:var(--radius-full);
}
.tag-core{background:rgba(34,211,238,0.12);color:var(--cyan)}
.tag-adv{background:rgba(245,197,66,0.12);color:var(--gold)}
.tag-mustknow{background:rgba(251,113,133,0.12);color:var(--rose)}
.tag-concept{background:rgba(124,106,255,0.12);color:var(--accent-light)}

.formula-thai{font-size:0.82em;color:var(--text-tertiary);font-weight:500;margin-bottom:10px;font-family:var(--font-thai)}

/* Formula expression */
.formula-expr{
  position:relative;
  background:var(--bg-code);
  color:var(--cyan);padding:16px 20px;border-radius:var(--radius-sm);
  font-family:var(--font-mono);font-size:0.87em;font-weight:400;
  margin:10px 0;overflow-x:auto;white-space:pre-wrap;word-break:break-all;
  line-height:1.8;border:1px solid var(--border-subtle);
}
.formula-concept-expr{
  position:relative;
  background:linear-gradient(135deg,rgba(124,106,255,0.15),rgba(124,106,255,0.05));
  border:1px solid rgba(124,106,255,0.2);
  color:var(--text-primary);padding:16px 20px;border-radius:var(--radius-sm);
  font-size:0.9em;font-weight:500;font-family:var(--font-thai);
  margin:10px 0;line-height:1.9;
}

.formula-vars{font-size:0.82em;color:var(--text-secondary);margin-top:10px;font-family:var(--font-thai)}
.formula-vars strong{color:var(--text-primary);font-weight:600}
.formula-vars ul{list-style:none;padding:4px 0 0}
.formula-vars li{padding:3px 0;display:flex;align-items:baseline;gap:8px}
.formula-vars li::before{content:"";width:4px;height:4px;border-radius:50%;background:var(--accent);flex-shrink:0;margin-top:8px}

.formula-seealso{font-size:0.75em;color:var(--text-tertiary);margin-top:8px;font-weight:500;font-family:var(--font-thai)}
.formula-seealso span{color:var(--accent-light)}

/* Card actions */
.card-actions{display:flex;gap:2px;flex-shrink:0}
.action-btn{
  width:32px;height:32px;display:inline-flex;align-items:center;justify-content:center;
  border:none;border-radius:var(--radius-xs);cursor:pointer;
  background:transparent;font-size:1em;transition:var(--transition);opacity:0.3;
}
.action-btn:hover{opacity:0.8;background:rgba(255,255,255,0.05)}
.action-btn.bookmarked{opacity:1;color:var(--rose);filter:drop-shadow(0 0 6px rgba(251,113,133,0.4))}
.action-btn.checked{opacity:1;color:var(--emerald);filter:drop-shadow(0 0 6px rgba(52,211,153,0.4))}

/* Copy button */
.copy-btn{
  position:absolute;top:10px;right:10px;
  display:inline-flex;align-items:center;gap:4px;
  background:rgba(255,255,255,0.05);border:1px solid var(--border);
  color:var(--text-tertiary);cursor:pointer;
  padding:4px 10px;border-radius:var(--radius-xs);
  font-size:0.72em;font-weight:500;font-family:var(--font-body);
  opacity:0;transition:var(--transition);
}
.formula-expr:hover .copy-btn,.formula-concept-expr:hover .copy-btn{opacity:1}
.copy-btn:hover{background:rgba(255,255,255,0.1);color:var(--text-secondary)}
.copy-btn.copied{color:var(--emerald)}

/* Example */
.formula-example{margin-top:12px}
.example-toggle{
  display:inline-flex;align-items:center;gap:6px;
  background:transparent;border:1.5px dashed rgba(255,255,255,0.1);
  color:var(--text-tertiary);padding:6px 16px;border-radius:var(--radius-full);
  cursor:pointer;font-size:0.78em;font-weight:600;font-family:var(--font-thai);
  transition:var(--transition);
}
.example-toggle:hover{border-color:var(--gold);color:var(--gold);border-style:solid;background:rgba(245,197,66,0.05)}
.example-content{
  display:none;margin-top:10px;padding:14px 18px;
  background:rgba(245,197,66,0.04);border-left:3px solid var(--gold);
  border-radius:0 var(--radius-sm) var(--radius-sm) 0;
  font-size:0.85em;font-family:var(--font-mono);font-weight:400;
  line-height:1.8;white-space:pre-wrap;color:var(--text-secondary);
}
.example-content.show{display:block;animation:fadeSlideUp 0.25s var(--ease)}

/* Highlight */
mark{background:rgba(245,197,66,0.2);color:var(--gold);padding:1px 3px;border-radius:3px}

/* ==========================================================
   BACK TO TOP
   ========================================================== */
.back-top{
  position:fixed;bottom:32px;right:32px;
  width:44px;height:44px;border-radius:var(--radius-md);
  background:var(--accent);color:#fff;border:none;
  font-size:1.1em;cursor:pointer;display:none;z-index:101;
  box-shadow:0 4px 20px var(--accent-glow);
  transition:0.3s var(--ease-spring);
  align-items:center;justify-content:center;
}
.back-top:hover{transform:translateY(-3px);box-shadow:0 8px 30px var(--accent-glow)}
.back-top svg{width:18px;height:18px}

/* ==========================================================
   TOAST
   ========================================================== */
.toast{
  position:fixed;bottom:90px;right:32px;
  background:var(--bg-elevated);color:var(--text-primary);
  border:1px solid var(--border);backdrop-filter:blur(16px);
  padding:10px 20px;border-radius:var(--radius-md);
  font-size:0.85em;font-weight:500;z-index:200;
  opacity:0;transform:translateY(8px) scale(0.96);
  transition:all 0.3s var(--ease);pointer-events:none;
  box-shadow:var(--shadow-lg);
}
.toast.show{opacity:1;transform:translateY(0) scale(1)}

/* ==========================================================
   CHEAT SHEET MODE
   ========================================================== */
body.cheatsheet .formula-thai,
body.cheatsheet .formula-vars,
body.cheatsheet .formula-example,
body.cheatsheet .formula-seealso,
body.cheatsheet .card-actions{display:none !important}
body.cheatsheet .formula-card{padding:10px 24px}
body.cheatsheet .formula-card::before{display:none}
body.cheatsheet .formula-expr,
body.cheatsheet .formula-concept-expr{padding:8px 14px;margin:4px 0;font-size:0.82em}
body.cheatsheet .formula-name{margin-bottom:0}

/* ==========================================================
   ANIMATIONS
   ========================================================== */
@keyframes fadeSlideUp{
  from{opacity:0;transform:translateY(-6px)}
  to{opacity:1;transform:translateY(0)}
}

/* ==========================================================
   PRINT
   ========================================================== */
@media print{
  body{background:#fff;color:#000}
  body::before,.ambient-orbs,.topbar,.controls-wrap,.subject-nav-wrap,.back-top,.toast,.card-actions,.copy-btn,.example-toggle,.progress-track,.kbd-hint{display:none !important}
  .subject-body.hidden,.category-body.hidden{display:block !important}
  .example-content{display:block !important;border-color:#ccc;background:#f9f9f9;color:#333}
  .formula-card{break-inside:avoid;border-bottom:1px solid #eee}
  .formula-expr{background:#f0f0f0 !important;color:#333;border:1px solid #ddd}
  .formula-concept-expr{background:#f5f0ff !important;color:#333;border:1px solid #ddd}
  .subject-header{print-color-adjust:exact;-webkit-print-color-adjust:exact}
  .content{padding:0;max-width:100%}
  .subject-section{opacity:1 !important;transform:none !important}
  .page-wrap{position:static}
}

/* ==========================================================
   RESPONSIVE — iPad (landscape & portrait)
   ========================================================== */
@media(max-width:1024px){
  .topbar{padding:16px 24px}
  .topbar-inner{gap:10px}
  .controls-wrap{padding:12px 24px}
  .controls{gap:8px}
  .subject-nav-wrap{padding:10px 24px}
  .content{padding:22px 24px 80px}
  .formula-expr,.formula-concept-expr{font-size:0.84em;padding:14px 16px}
}

/* ==========================================================
   RESPONSIVE — Mobile (phones)
   ========================================================== */
@media(max-width:768px){
  /* Base sizing */
  body{font-size:14px}

  /* Topbar — compact */
  .topbar{padding:12px 16px}
  .topbar-brand h1{font-size:1.3em}
  .topbar-brand p{font-size:0.72em}
  .topbar-actions{gap:4px}
  .topbar-btn{padding:6px 10px;font-size:0.72em;gap:4px}
  .topbar-btn svg{width:12px;height:12px}

  /* Controls — stack on mobile */
  .controls-wrap{padding:10px 16px;top:54px}
  .controls{gap:8px}
  .search-wrap{min-width:100%;order:-1}
  .search-input{padding:10px 14px 10px 38px;font-size:0.88em}
  .filter-select{padding:8px 30px 8px 10px;font-size:0.82em;flex:1;min-width:0}
  .view-pills{flex:1;min-width:0}
  .view-pill{padding:6px 10px;font-size:0.76em}
  .stats-chip{font-size:0.72em;padding:4px 10px}
  .kbd-hint{display:none}

  /* Subject nav — horizontal scroll, smaller chips */
  .subject-nav-wrap{padding:8px 16px}
  .subject-nav{gap:4px}
  .subject-chip{padding:6px 10px;font-size:0.72em;gap:4px}
  .subject-chip .color-dot{width:6px;height:6px}
  .chip-badge{font-size:0.7em;padding:1px 5px}

  /* Content */
  .content{padding:14px 12px 60px}

  /* Subject headers */
  .subject-section{margin-bottom:20px}
  .subject-header{padding:14px 16px;border-radius:var(--radius-sm) var(--radius-sm) 0 0}
  .subject-header h2{font-size:0.88em}
  .subject-count{font-size:0.7em;padding:2px 8px}
  .subject-body{border-radius:0 0 var(--radius-sm) var(--radius-sm)}

  /* Category */
  .category-header{padding:10px 16px}
  .category-title{font-size:0.82em}

  /* Formula cards — the most important part on mobile */
  .formula-card{padding:14px 14px}
  .formula-name{gap:4px}
  .formula-name-text{font-size:0.88em}
  .tag{font-size:0.55em;padding:2px 6px}
  .formula-thai{font-size:0.78em;margin-bottom:8px}
  .formula-expr,.formula-concept-expr{
    padding:12px 14px;font-size:0.8em;line-height:1.7;
    margin:6px 0;border-radius:var(--radius-xs);
    /* Enable horizontal scroll for long formulas */
    overflow-x:auto;-webkit-overflow-scrolling:touch;
  }
  .formula-vars{font-size:0.78em}
  .formula-vars li{padding:2px 0;gap:6px}
  .formula-seealso{font-size:0.7em}
  .example-toggle{font-size:0.74em;padding:5px 12px}
  .example-content{font-size:0.78em;padding:10px 14px}

  /* Action buttons — bigger touch targets */
  .action-btn{width:36px;height:36px;font-size:1.1em}
  .card-actions{gap:0}
  .copy-btn{opacity:0.6;font-size:0.68em;padding:3px 8px}

  /* Back to top */
  .back-top{bottom:20px;right:20px;width:40px;height:40px}
  .back-top svg{width:16px;height:16px}

  /* Toast */
  .toast{bottom:70px;right:16px;left:16px;text-align:center;font-size:0.82em}

  /* Orbs — reduce for performance */
  .orb{opacity:0.04}
  .orb-1{width:300px;height:300px}
  .orb-2{width:250px;height:250px}
  .orb-3{width:200px;height:200px}
}

/* ==========================================================
   RESPONSIVE — Very small phones (iPhone SE, etc.)
   ========================================================== */
@media(max-width:375px){
  .topbar-brand h1{font-size:1.15em}
  .topbar-btn span:not(#themeLabel){display:none}
  .topbar-btn{padding:6px 8px}
  .view-pill{padding:5px 8px;font-size:0.72em}
  .filter-select{font-size:0.78em}
  .formula-name-text{font-size:0.84em}
  .formula-expr,.formula-concept-expr{font-size:0.76em;padding:10px 12px}
}

/* ==========================================================
   TOUCH & MOBILE UX ENHANCEMENTS
   ========================================================== */
/* Prevent text selection on interactive elements */
.topbar-btn,.view-pill,.subject-chip,.action-btn,.example-toggle,.category-header,.subject-header{
  -webkit-tap-highlight-color:transparent;
  touch-action:manipulation;
}

/* Safe area insets for notched phones */
@supports(padding:max(0px)){
  .topbar{padding-left:max(16px,env(safe-area-inset-left));padding-right:max(16px,env(safe-area-inset-right))}
  .controls-wrap{padding-left:max(16px,env(safe-area-inset-left));padding-right:max(16px,env(safe-area-inset-right))}
  .content{padding-left:max(12px,env(safe-area-inset-left));padding-right:max(12px,env(safe-area-inset-right))}
  .toast{left:max(16px,env(safe-area-inset-left));right:max(16px,env(safe-area-inset-right));bottom:max(70px,calc(env(safe-area-inset-bottom) + 20px))}
  .back-top{bottom:max(20px,calc(env(safe-area-inset-bottom) + 10px));right:max(20px,env(safe-area-inset-right))}
}

/* Landscape phone — reduce sticky header height */
@media(max-width:768px) and (orientation:landscape){
  .topbar{padding:8px 16px}
  .topbar-brand p{display:none}
  .controls-wrap{top:42px;padding:8px 16px}
  .view-pills{display:none}
}

/* ==========================================================
   LIGHT MODE
   ========================================================== */
body.light{
  --bg-base:#f4f4f8;--bg-surface:rgba(255,255,255,0.85);--bg-surface-solid:#fff;
  --bg-elevated:rgba(255,255,255,0.9);--bg-card:rgba(255,255,255,0.7);--bg-card-hover:rgba(240,238,255,0.7);
  --text-primary:#1a1a2e;--text-secondary:#5c5c7a;--text-tertiary:#9d9db8;
  --border:rgba(0,0,0,0.08);--border-subtle:rgba(0,0,0,0.04);--border-hover:rgba(0,0,0,0.15);
  --accent-glow:rgba(124,106,255,0.15);--cyan-glow:rgba(34,211,238,0.1);
}
body.light::before{opacity:0.015}
body.light .orb{opacity:0.06}
body.light .topbar{background:rgba(244,244,248,0.8)}
body.light .controls-wrap{background:rgba(244,244,248,0.75)}
body.light .search-input{background:rgba(0,0,0,0.03)}
body.light .search-input:focus{background:rgba(0,0,0,0.02)}
body.light .filter-select{background:rgba(0,0,0,0.03)}
body.light .filter-select option{background:#fff}
body.light .view-pills{background:rgba(0,0,0,0.03)}
body.light .subject-nav-wrap{background:rgba(244,244,248,0.6)}
body.light .subject-chip{background:rgba(0,0,0,0.02);border-color:rgba(0,0,0,0.08)}
body.light .subject-chip:hover{background:rgba(124,106,255,0.06)}
body.light .subject-body{background:rgba(255,255,255,0.8);border-color:rgba(0,0,0,0.06)}
body.light .category-header{background:rgba(0,0,0,0.02)}
body.light .category-header:hover{background:rgba(124,106,255,0.04)}
body.light .formula-card:hover{background:rgba(124,106,255,0.04)}
body.light .action-btn:hover{background:rgba(0,0,0,0.04)}
body.light .formula-concept-expr{background:linear-gradient(135deg,rgba(124,106,255,0.08),rgba(124,106,255,0.03))}
body.light .example-content{background:rgba(245,197,66,0.06);color:var(--text-primary)}
body.light mark{background:rgba(245,197,66,0.25);color:#7c5e00}
body.light .toast{background:var(--bg-surface);border-color:var(--border)}
body.light .kbd{background:rgba(0,0,0,0.04);border-color:rgba(0,0,0,0.1)}
body.light .chip-badge{background:rgba(0,0,0,0.05)}
body.light .stats-chip{background:rgba(0,0,0,0.03)}
body.light .topbar-btn{background:rgba(0,0,0,0.03);border-color:rgba(0,0,0,0.08);color:var(--text-secondary)}
body.light .topbar-btn:hover{background:rgba(0,0,0,0.06);color:var(--text-primary)}