:root{
  --bg:#f6f7f9; --card:#ffffff; --ink:#15202b; --muted:#5b6775; --line:#e3e7ec;
  --navy:#0f2a44; --navy-2:#173a5e; --accent:#e8792b; --accent-2:#f3a05f;
  --ok:#1e8e5a; --ok-bg:#e6f6ee; --bad:#c43d3d; --bad-bg:#fbe9e9; --warn-bg:#fff6e5;
  --radius:14px; --shadow:0 1px 2px rgba(15,42,68,.06),0 8px 24px rgba(15,42,68,.07);
  --font:"Heebo",-apple-system,BlinkMacSystemFont,"Segoe UI",Roboto,Arial,sans-serif;
}
@media (prefers-color-scheme: dark){ :root:not([data-theme="light"]){
  --bg:#0f1620; --card:#18222e; --ink:#eef2f6; --muted:#9aa8b8; --line:#2a3645;
  --navy:#0b1e33; --navy-2:#13304f; --ok-bg:#153527; --bad-bg:#3a1f1f; --warn-bg:#3a2d14;
  --shadow:0 1px 2px rgba(0,0,0,.3),0 8px 24px rgba(0,0,0,.35);
}}
*{box-sizing:border-box}
html{-webkit-text-size-adjust:100%}
body{margin:0;background:var(--bg);color:var(--ink);font-family:var(--font);font-size:17px;line-height:1.55;min-height:100vh;display:flex;flex-direction:column}
a{color:inherit}
button{font:inherit;cursor:pointer}
.hidden{display:none !important}

/* header */
.top{position:sticky;top:0;z-index:20;background:var(--navy);color:#fff;display:flex;align-items:center;gap:12px;padding:10px 16px;padding-top:max(10px,env(safe-area-inset-top))}
.brand{font-weight:800;font-size:19px;text-decoration:none;white-space:nowrap}
.nav{display:flex;gap:4px;margin-inline-start:auto;overflow-x:auto;scrollbar-width:none}
.nav::-webkit-scrollbar{display:none}
.nav a{color:#dbe6f3;text-decoration:none;padding:6px 10px;border-radius:999px;font-size:15px;white-space:nowrap}
.nav a.active{background:rgba(255,255,255,.14);color:#fff}
.lock{background:transparent;border:1px solid rgba(255,255,255,.3);color:#fff;border-radius:999px;width:36px;height:36px;display:grid;place-items:center;flex:none}
.lock.open{background:var(--accent);border-color:var(--accent)}

/* layout */
.app{flex:1;width:100%;max-width:860px;margin:0 auto;padding:16px 16px 40px}
.foot{text-align:center;color:var(--muted);font-size:13px;padding:20px 16px 28px;border-top:1px solid var(--line)}
.loading{text-align:center;color:var(--muted);padding:40px}
h1{font-size:28px;line-height:1.2;margin:8px 0 10px;font-weight:800}
h2{font-size:21px;margin:22px 0 10px;font-weight:700}
h3{font-size:17px;margin:14px 0 6px;font-weight:700}
p{margin:6px 0}
.muted{color:var(--muted)}
.small{font-size:14px}
.card{background:var(--card);border:1px solid var(--line);border-radius:var(--radius);box-shadow:var(--shadow);padding:16px;margin:12px 0}
.row{display:flex;gap:10px;flex-wrap:wrap;align-items:center}
.grid{display:grid;gap:12px;grid-template-columns:repeat(auto-fill,minmax(230px,1fr))}
.btn{display:inline-flex;align-items:center;justify-content:center;gap:8px;background:var(--navy);color:#fff;border:0;border-radius:12px;padding:12px 18px;font-weight:700;text-decoration:none;min-height:46px}
.btn.accent{background:var(--accent)}
.btn.ghost{background:transparent;color:var(--ink);border:1px solid var(--line)}
.btn.sm{padding:8px 12px;min-height:38px;font-size:15px;border-radius:10px}
.btn.block{width:100%}
.btn:disabled{opacity:.5;cursor:default}
.chip{display:inline-flex;align-items:center;gap:6px;border:1px solid var(--line);background:var(--card);border-radius:999px;padding:6px 12px;font-size:15px;cursor:pointer;user-select:none}
.chip.on{background:var(--navy);color:#fff;border-color:var(--navy)}
.chip .n{opacity:.7;font-size:13px}
.tag{display:inline-block;background:var(--warn-bg);color:var(--ink);border-radius:8px;padding:2px 8px;font-size:13px;margin-inline-end:6px}
.tag.lock{background:var(--bad-bg)}
.tag.free{background:var(--ok-bg)}
.pill{display:inline-block;border-radius:999px;padding:2px 10px;font-size:13px;background:var(--line)}

/* hero */
.hero{background:linear-gradient(135deg,var(--navy),var(--navy-2));color:#fff;border-radius:18px;padding:26px 20px;margin-bottom:16px}
.hero h1{color:#fff;margin-top:0}
.hero p{color:#dbe6f3;font-size:17px}
.stats{display:grid;grid-template-columns:repeat(3,1fr);gap:10px;margin:16px 0 6px}
.stat{background:rgba(255,255,255,.1);border-radius:12px;padding:10px;text-align:center}
.stat b{display:block;font-size:24px;line-height:1.1}
.stat span{font-size:13px;color:#dbe6f3}
.hero .row{margin-top:14px}
.feature{display:flex;gap:12px;align-items:flex-start}
.feature .ic{font-size:26px;line-height:1;flex:none}

/* question */
.q-head{display:flex;justify-content:space-between;align-items:center;gap:10px;flex-wrap:wrap;margin-bottom:8px}
.q-stem{font-size:17px}
.q-stem table,.md table{border-collapse:collapse;margin:10px 0;width:100%;font-size:15px}
.q-stem th,.q-stem td,.md th,.md td{border:1px solid var(--line);padding:6px 8px;text-align:right;vertical-align:top}
.q-stem th,.md th{background:var(--bg)}
.md ul,.md ol{padding-inline-start:22px;margin:6px 0}
.md p{margin:6px 0}
.md pre{white-space:pre-wrap;font-family:inherit;margin:4px 0}
.opts{display:grid;gap:8px;margin:14px 0}
.opt{display:flex;gap:10px;align-items:flex-start;text-align:right;background:var(--card);border:1.5px solid var(--line);border-radius:12px;padding:12px 14px;width:100%;color:var(--ink)}
.opt .l{flex:none;width:28px;height:28px;border-radius:50%;background:var(--bg);display:grid;place-items:center;font-weight:700}
.opt:hover{border-color:var(--navy)}
.opt.sel{border-color:var(--navy);background:var(--bg)}
.opt.ok{border-color:var(--ok);background:var(--ok-bg)}
.opt.ok .l{background:var(--ok);color:#fff}
.opt.bad{border-color:var(--bad);background:var(--bad-bg)}
.opt.bad .l{background:var(--bad);color:#fff}
.opt:disabled{cursor:default}
.verdict{border-radius:12px;padding:12px 14px;margin:10px 0;font-weight:700}
.verdict.ok{background:var(--ok-bg);color:var(--ok)}
.verdict.bad{background:var(--bad-bg);color:var(--bad)}
.sol{border-top:1px dashed var(--line);margin-top:12px;padding-top:12px}
.sol details{margin:8px 0}
.sol summary{cursor:pointer;font-weight:700}
.distractors li{margin:4px 0}
.progressbar{height:8px;background:var(--line);border-radius:999px;overflow:hidden;margin:8px 0}
.progressbar i{display:block;height:100%;background:var(--accent);border-radius:999px;transition:width .3s}
.nextbar{display:flex;gap:10px;justify-content:space-between;align-items:center;margin-top:14px;flex-wrap:wrap}

/* exam sim */
.simbar{position:sticky;top:58px;z-index:10;background:var(--card);border:1px solid var(--line);border-radius:12px;padding:8px 12px;display:flex;align-items:center;gap:10px;flex-wrap:wrap;margin-bottom:10px}
.timer{font-variant-numeric:tabular-nums;font-weight:800;font-size:18px}
.timer.low{color:var(--bad)}
.qnav{display:flex;gap:6px;flex-wrap:wrap}
.qnav button{width:34px;height:34px;border-radius:8px;border:1px solid var(--line);background:var(--card);color:var(--ink);font-weight:700}
.qnav button.cur{border-color:var(--navy);outline:2px solid var(--navy)}
.qnav button.done{background:var(--navy);color:#fff;border-color:var(--navy)}
.qnav button.ok{background:var(--ok);color:#fff;border-color:var(--ok)}
.qnav button.bad{background:var(--bad);color:#fff;border-color:var(--bad)}
.score{font-size:44px;font-weight:800;line-height:1}

/* flashcards */
.fc{perspective:1200px;margin:10px 0}
.fc .inner{position:relative;min-height:260px;transition:transform .45s;transform-style:preserve-3d}
.fc.flipped .inner{transform:rotateY(180deg)}
.fc .face{position:absolute;inset:0;backface-visibility:hidden;-webkit-backface-visibility:hidden;background:var(--card);border:1px solid var(--line);border-radius:18px;box-shadow:var(--shadow);padding:22px;display:flex;flex-direction:column;justify-content:center;align-items:center;text-align:center;font-size:19px;cursor:pointer}
.fc .face.back{transform:rotateY(180deg);font-size:17px;text-align:right;align-items:stretch;justify-content:flex-start;overflow:auto}
.fc .face .k{position:absolute;top:10px;right:14px;font-size:12px;color:var(--muted)}
.rate{display:grid;grid-template-columns:1fr 1fr;gap:10px;margin-top:12px}
.boxes{display:flex;gap:6px;margin:8px 0}
.boxes span{flex:1;text-align:center;background:var(--line);border-radius:8px;padding:4px 0;font-size:13px}

/* stats */
.bar{display:flex;align-items:center;gap:10px;margin:6px 0}
.bar .lbl{width:150px;flex:none;font-size:15px}
.bar .trk{flex:1;height:12px;background:var(--line);border-radius:999px;overflow:hidden}
.bar .trk i{display:block;height:100%;background:var(--ok)}
.bar .trk i.weak{background:var(--bad)}
.bar .v{width:64px;text-align:left;font-size:14px;color:var(--muted);direction:ltr}

/* modal */
.modal{position:fixed;inset:0;background:rgba(10,20,30,.7);z-index:50;display:flex;align-items:flex-start;justify-content:center;padding:14px;overflow:auto}
.modal .box{background:var(--card);border-radius:16px;max-width:900px;width:100%;padding:16px;margin:auto 0}
.modal img{width:100%;height:auto;border-radius:8px;margin:8px 0;background:#fff}
.modal .x{position:sticky;top:0;display:flex;justify-content:space-between;align-items:center;margin-bottom:8px}
input.code{font:inherit;font-size:20px;letter-spacing:1px;direction:ltr;text-align:center;width:100%;padding:12px;border:1.5px solid var(--line);border-radius:12px;background:var(--bg);color:var(--ink)}
.err{color:var(--bad);font-weight:700}
.price{font-size:40px;font-weight:800;line-height:1}
.price small{font-size:18px;font-weight:500}
ul.checks{list-style:none;padding:0;margin:10px 0}
ul.checks li{padding-inline-start:28px;position:relative;margin:6px 0}
ul.checks li::before{content:"✓";position:absolute;right:0;color:var(--ok);font-weight:800}
.notice{background:var(--warn-bg);border-radius:12px;padding:10px 14px;margin:10px 0}
.examlist .card{display:flex;justify-content:space-between;align-items:center;gap:10px;flex-wrap:wrap;margin:8px 0;padding:12px 14px}
@media (max-width:520px){
  body{font-size:16px}
  h1{font-size:24px}
  .stats{gap:6px}
  .stat b{font-size:20px}
  .bar .lbl{width:110px}
  .simbar{top:56px}
}

/* learn */
.back{display:inline-block;color:var(--muted);text-decoration:none;font-size:15px;margin-bottom:4px}
.tgrid{display:grid;gap:10px;grid-template-columns:repeat(auto-fill,minmax(250px,1fr))}
.tcard{text-decoration:none;display:block;margin:0}
.tcard-head{display:flex;justify-content:space-between;align-items:center;gap:8px;font-size:18px}
.clist{margin:12px 0}
.citem{display:flex;gap:12px;align-items:center;background:var(--card);border:1px solid var(--line);border-radius:12px;padding:10px 12px;margin:6px 0;text-decoration:none;color:var(--ink)}
.citem .num{flex:none;width:30px;height:30px;border-radius:50%;background:var(--bg);display:grid;place-items:center;font-weight:700;font-size:14px}
.citem.done .num{background:var(--ok);color:#fff}
.citem.locked{opacity:.55}
.reader{max-width:640px;margin:0 auto}
.lcard{background:var(--card);border:1px solid var(--line);border-radius:18px;box-shadow:var(--shadow);padding:22px 20px;margin:12px 0;font-size:17px;line-height:1.65;touch-action:pan-y}
.lcard h2{margin:0 0 12px;font-size:23px}
.lcard .lbl{display:block;font-size:12px;font-weight:700;color:var(--muted);letter-spacing:.3px;margin-bottom:4px}
.formula,.example,.trap{border-radius:12px;padding:10px 14px;margin:12px 0}
.formula{background:var(--bg);font-weight:600;font-size:16px}
.example{background:var(--ok-bg)}
.trap{background:var(--warn-bg)}
.src{font-size:12px;margin-top:12px}
@media (max-width:520px){.lcard{padding:18px 16px}}

/* open-access review mode */
.banner{background:var(--warn-bg);color:var(--ink);text-align:center;padding:8px 16px;font-size:14px;border-bottom:1px solid var(--line)}
.banner a{font-weight:700}
.fab{position:fixed;bottom:max(16px,env(safe-area-inset-bottom));left:16px;z-index:30;background:var(--accent);color:#fff;text-decoration:none;font-weight:700;border-radius:999px;padding:12px 18px;box-shadow:0 6px 20px rgba(0,0,0,.25)}