:root{
  --ink:#0e1a30; --ink-soft:#46566f; --ink-mute:#7c889b;
  --bg:#ffffff; --card:#ffffff; --soft:#f5f7fb; --line:#e6ebf3;
  --brand:#2f4bd0; --brand-deep:#2138b0; --brand-tint:#eef1fd;
  --hi:#f0a91b; --ok:#16a34a;
  --r:16px; --r-sm:11px;
  --shadow:0 1px 2px rgba(16,30,55,.05), 0 8px 26px rgba(16,30,55,.06);
  --shadow-h:0 2px 6px rgba(16,30,55,.08), 0 18px 44px rgba(16,30,55,.12);
}
*{box-sizing:border-box}
html{scroll-behavior:smooth;-webkit-text-size-adjust:100%}
body{
  margin:0;color:var(--ink);background:var(--bg);
  font-family:"Inter",system-ui,-apple-system,Segoe UI,Roboto,sans-serif;
  font-size:16px;line-height:1.6;-webkit-font-smoothing:antialiased;
}
.wrap{max-width:1120px;margin:0 auto;padding:0 18px}
a{color:var(--brand)}
:focus-visible{outline:3px solid var(--brand);outline-offset:2px;border-radius:8px}

/* header */
.top{position:sticky;top:0;z-index:40;background:rgba(255,255,255,.86);backdrop-filter:blur(12px);border-bottom:1px solid var(--line)}
.top .wrap{display:flex;align-items:center;gap:14px;min-height:62px}
.brand{display:flex;align-items:center;gap:11px;text-decoration:none;color:var(--ink);cursor:pointer;background:none;border:0;padding:0}
.mark{width:38px;height:38px;border-radius:11px;background:linear-gradient(150deg,var(--brand),#4a63e0);display:flex;align-items:center;justify-content:center;flex:none;box-shadow:0 6px 16px rgba(47,75,208,.32)}
.mark span{font-family:"Fraunces",serif;font-weight:600;color:#fff;font-size:18px}
.brand b{font-family:"Fraunces",serif;font-weight:600;font-size:23px;letter-spacing:-.01em;line-height:1}
.brand i{font-style:normal;color:var(--brand);font-weight:600}
.top .meta{margin-left:auto;font-family:"JetBrains Mono",monospace;font-size:12.5px;color:var(--ink-mute)}
.top .meta b{color:var(--brand);font-weight:600}

/* hero */
.hero{padding:44px 0 24px;text-align:center}
.eyebrow{font-family:"JetBrains Mono",monospace;font-size:12px;letter-spacing:.18em;text-transform:uppercase;color:var(--ink-mute);margin-bottom:20px}
.featured{min-height:70px;display:flex;align-items:center;justify-content:center;margin:0 auto 12px;max-width:100%;overflow-x:auto;transition:opacity .25s ease}
.featured .katex{font-size:2.4em;color:var(--ink)}
.hero h1{font-family:"Fraunces",serif;font-weight:600;font-size:clamp(24px,4.6vw,40px);line-height:1.12;letter-spacing:-.02em;margin:6px auto;max-width:18ch}
.hero h1 .u{background:linear-gradient(transparent 62%, #ffe6a8 62%);padding:0 .06em}
.hero .sub{color:var(--ink-soft);max-width:52ch;margin:0 auto 24px;font-size:17px}

/* search */
.search{position:relative;max-width:620px;margin:0 auto}
.search svg.ic{position:absolute;left:18px;top:50%;transform:translateY(-50%);color:var(--ink-mute);pointer-events:none}
#q{width:100%;font-family:"Inter",sans-serif;font-size:17px;color:var(--ink);padding:17px 92px 17px 50px;border:1.5px solid var(--line);border-radius:14px;background:var(--card);box-shadow:var(--shadow);transition:border-color .15s, box-shadow .15s}
#q::placeholder{color:var(--ink-mute)}
#q:focus{outline:none;border-color:var(--brand);box-shadow:0 0 0 4px var(--brand-tint),var(--shadow)}
.kbd{position:absolute;right:48px;top:50%;transform:translateY(-50%);font-family:"JetBrains Mono",monospace;font-size:12px;color:var(--ink-mute);border:1px solid var(--line);border-bottom-width:2px;border-radius:7px;padding:2px 8px;background:var(--soft);pointer-events:none}
#clear{position:absolute;right:10px;top:50%;transform:translateY(-50%);width:30px;height:30px;border:0;border-radius:8px;background:transparent;color:var(--ink-mute);cursor:pointer;display:none;align-items:center;justify-content:center}
#clear:hover{background:var(--brand-tint);color:var(--brand)}
.search.has #clear{display:flex}.search.has .kbd{display:none}

/* chips */
.chips{display:flex;gap:9px;overflow-x:auto;padding:20px 18px 4px;margin:0 -18px;scrollbar-width:none;-webkit-overflow-scrolling:touch;justify-content:flex-start}
.chips::-webkit-scrollbar{display:none}
.chip{flex:none;display:inline-flex;align-items:center;gap:7px;font-size:14px;font-weight:500;color:var(--ink-soft);background:var(--card);border:1.5px solid var(--line);border-radius:999px;padding:8px 15px;cursor:pointer;white-space:nowrap;transition:all .14s}
.chip:hover{border-color:#c8d3e6}
.chip .dot{width:8px;height:8px;border-radius:50%;background:var(--brand)}
.chip.on{background:var(--ink);color:#fff;border-color:var(--ink)}

/* section heading */
.sec-h{display:flex;align-items:baseline;justify-content:space-between;gap:12px;padding:26px 0 4px;flex-wrap:wrap}
.sec-h h2{font-family:"Fraunces",serif;font-weight:600;font-size:22px;letter-spacing:-.01em;margin:0;display:flex;align-items:center;gap:10px}
.sec-h h2 .cdot{width:11px;height:11px;border-radius:50%}
.sec-h .lnk{font-size:14px;color:var(--brand);background:none;border:0;cursor:pointer;font-weight:600}
.count{font-size:14px;color:var(--ink-mute);padding:2px 0 2px}
.count b{color:var(--ink);font-weight:600}

/* category tiles */
.tiles{display:grid;grid-template-columns:repeat(auto-fit,minmax(244px,1fr));gap:16px;padding:8px 0 6px}
.tile{text-align:left;background:var(--card);border:1px solid var(--line);border-radius:var(--r);padding:18px;cursor:pointer;display:flex;flex-direction:column;gap:10px;box-shadow:var(--shadow);transition:transform .14s, box-shadow .14s}
.tile:hover{transform:translateY(-3px);box-shadow:var(--shadow-h)}
.tile .ic{align-self:flex-start;border-radius:10px;padding:8px 12px;min-height:40px;display:flex;align-items:center}
.tile .ic .katex{font-size:1.15em}
.tile .tn{font-family:"Fraunces",serif;font-weight:600;font-size:19px;display:flex;align-items:center;justify-content:space-between;gap:8px}
.tile .tn .ar{color:var(--ink-mute);transition:transform .14s}
.tile:hover .tn .ar{transform:translateX(3px)}
.tile .tc{font-size:13px;color:var(--ink-mute);font-family:"JetBrains Mono",monospace}
.tile .ts{font-size:13px;color:var(--ink-soft);line-height:1.45}

/* grid + cards */
.grid{display:grid;grid-template-columns:repeat(auto-fill,minmax(310px,1fr));gap:16px;padding:6px 0 40px}
.card{background:var(--card);border:1px solid var(--line);border-radius:var(--r);padding:18px 18px 16px;box-shadow:var(--shadow);display:flex;flex-direction:column;gap:11px;transition:transform .14s, box-shadow .14s}
.card:hover{transform:translateY(-3px);box-shadow:var(--shadow-h)}
.c-top{display:flex;align-items:center;gap:8px}
.tag{display:inline-flex;align-items:center;gap:7px;font-size:12px;font-weight:600;color:var(--ink-soft)}
.tag .dot{width:9px;height:9px;border-radius:50%}
.copy{margin-left:auto;display:inline-flex;align-items:center;gap:6px;font-size:12px;font-family:"JetBrains Mono",monospace;color:var(--ink-mute);background:var(--soft);border:1px solid var(--line);border-radius:8px;padding:5px 9px;cursor:pointer;transition:all .14s}
.copy:hover{color:var(--brand);border-color:#c2cef0;background:var(--brand-tint)}
.copy.done{color:var(--ok);border-color:#bbe6c7;background:#edf9f1}
.c-name{font-weight:600;font-size:16.5px;letter-spacing:-.01em;line-height:1.25}
.f-math{background:var(--soft);border:1px solid var(--line);border-radius:var(--r-sm);padding:14px 12px;overflow-x:auto;overflow-y:hidden;text-align:center}
.f-math .katex{font-size:1.32em}
.c-desc{font-size:14px;color:var(--ink-soft);margin:0}
.vars{display:flex;flex-wrap:wrap;gap:6px;margin-top:auto;padding-top:3px}
.var{font-size:12px;color:var(--ink-soft);background:var(--soft);border:1px solid var(--line);border-radius:7px;padding:3px 9px}
.var b{font-family:"JetBrains Mono",monospace;color:var(--ink);font-weight:600}

/* empty */
.empty{display:none;text-align:center;padding:50px 18px 64px}
.empty.show{display:block}
.empty .big{font-family:"Fraunces",serif;font-size:24px;font-weight:600;margin-bottom:8px}
.empty p{color:var(--ink-soft);margin:0 auto;max-width:40ch}
.empty button{margin-top:18px;font:inherit;font-weight:600;color:#fff;background:var(--brand);border:0;border-radius:11px;padding:11px 20px;cursor:pointer}
.empty button:hover{background:var(--brand-deep)}

/* footer */
footer{border-top:1px solid var(--line);background:var(--soft);margin-top:10px}
footer .wrap{padding:30px 18px;display:flex;justify-content:space-between;gap:18px;flex-wrap:wrap;align-items:center}
footer .fb{display:flex;align-items:center;gap:10px}
footer .fb .mark{width:30px;height:30px;border-radius:9px}
footer b{font-family:"Fraunces",serif;font-weight:600;font-size:17px}
footer small{color:var(--ink-mute);font-size:13px;max-width:46ch}

@media(max-width:560px){
  .hero{padding:32px 0 16px}
  .featured .katex{font-size:1.95em}
  .hero .sub{font-size:15.5px}
  #q{font-size:16px;padding:15px 16px 15px 46px}
  .kbd{display:none}
  .grid{grid-template-columns:1fr}
  .tiles{grid-template-columns:1fr}
  .top .meta{display:none}
}
@media(prefers-reduced-motion:reduce){*{transition:none!important;scroll-behavior:auto!important}}

/* ============ Mobile responsiveness (enhanced) ============ */
html{scroll-padding-top:74px}
body{overflow-x:clip}
*{-webkit-tap-highlight-color:rgba(47,75,208,.12)}
/* wide formulas scroll inside their box instead of stretching the layout */
.grid,.tiles,.card,.tile{min-width:0}
.f-math,.featured{max-width:100%}
.tile .tn .ar{flex:none}
/* long unbreakable strings never force horizontal scroll */
.hero h1,.c-name,.tile .tn,.sec-h h2,.tag,.var,.eyebrow,.tile .tc{overflow-wrap:anywhere}

/* ---- Tablets ---- */
@media(max-width:820px){
  .hero{padding:38px 0 20px}
  .tiles,.grid{gap:14px}
}

/* ---- Phones ---- */
@media(max-width:600px){
  .wrap{padding:0 15px}
  .chips{padding:16px 15px 4px;margin:0 -15px}
  .top .wrap{gap:10px;min-height:56px}
  .brand b{font-size:20px}
  .mark{width:34px;height:34px;border-radius:10px}
  .mark span{font-size:16px}
  .top .meta{display:none}
  .hero{padding:28px 0 14px}
  .hero h1{font-size:clamp(22px,6.4vw,30px)}
  .hero .sub{font-size:15.5px}
  .eyebrow{margin-bottom:14px;font-size:11px}
  .featured .katex{font-size:1.8em}
  .sec-h{padding:22px 0 2px}
  .sec-h h2{font-size:20px}
  .tiles,.grid{grid-template-columns:1fr;gap:12px}
  .tile{padding:16px}
  .card{padding:16px 15px 14px}
  .f-math{padding:12px 10px}
  .f-math .katex{font-size:1.18em}
  /* comfortable tap targets */
  .chip{padding:10px 15px}
  .copy{padding:7px 11px}
  #clear{width:34px;height:34px;right:8px}
  #q{font-size:16px;padding:15px 48px 15px 46px}
  .kbd{display:none}
  .sec-h .lnk{padding:4px 0}
}

/* ---- Small phones ---- */
@media(max-width:400px){
  .wrap{padding:0 12px}
  .chips{padding:14px 12px 4px;margin:0 -12px}
  .hero h1{font-size:clamp(20px,7vw,26px)}
  .hero .sub{font-size:15px}
  .brand b{font-size:19px}
  .tile .tn{font-size:18px}
  .c-name{font-size:16px}
  .f-math .katex{font-size:1.08em}
  .featured .katex{font-size:1.6em}
  footer .wrap{padding:24px 12px}
}

/* ---- Short landscape screens ---- */
@media(max-height:480px) and (orientation:landscape){
  .hero{padding:20px 0 10px}
  .top{position:static}
}