:root{
  --bg:#f4f3ff; --bg2:#eef4ff;
  --ink:#1f2430; --muted:#6b7280; --line:#e7e7f3;
  --card:#ffffff;
  --brand:#6d5efc; --brand-d:#5848e0;
  --accent:#ff8a3d; --accent-d:#f06f1e;
  --good:#0f9d58; --good-bg:#e7f7ef;
  --bad:#e5484d; --bad-bg:#fdecec;
  --hint:#0d9488; --hint-bg:#e6f6f4;
  --r:18px; --sh:0 6px 22px rgba(60,50,140,.10);
}
*{box-sizing:border-box}
html,body{margin:0;padding:0}
body{
  font-family:system-ui,-apple-system,"Segoe UI",Roboto,Arial,sans-serif;
  color:var(--ink);
  background:linear-gradient(160deg,var(--bg),var(--bg2));
  background-attachment:fixed;
  min-height:100vh;
  -webkit-text-size-adjust:100%;
  line-height:1.5;
}
.app{max-width:780px;margin:0 auto;padding:16px 16px 40px}
.loading{padding:60px 0;text-align:center;color:var(--muted)}

/* top bar */
.topbar{
  position:sticky;top:0;z-index:20;
  display:flex;align-items:center;gap:10px;
  padding:max(10px,env(safe-area-inset-top)) 14px 10px;
  background:rgba(255,255,255,.82);backdrop-filter:blur(10px);
  border-bottom:1px solid var(--line);
}
.brand{font-weight:800;color:var(--brand-d);text-decoration:none;font-size:1.05rem;letter-spacing:.2px}
.icon-btn{
  border:none;background:#fff;color:var(--brand-d);
  width:34px;height:34px;border-radius:50%;font-size:1.5rem;line-height:1;
  box-shadow:var(--sh);cursor:pointer;font-weight:800;
}
.icon-btn:active{transform:scale(.94)}
.pill{margin-left:auto;background:var(--brand);color:#fff;font-weight:700;
  font-size:.82rem;padding:5px 12px;border-radius:999px}
.foot{text-align:center;color:var(--muted);font-size:.8rem;padding:8px 16px 26px}

/* ---------- menu ---------- */
.hero{
  background:linear-gradient(135deg,var(--brand),#8b7bff);
  color:#fff;border-radius:var(--r);padding:20px 20px 22px;margin:6px 0 18px;
  box-shadow:var(--sh);
}
.hero h1{margin:0 0 4px;font-size:1.5rem}
.hero p{margin:0;opacity:.92;font-size:.95rem}
.hero .bar{margin-top:14px;background:rgba(255,255,255,.25);border-radius:999px;height:9px;overflow:hidden}
.hero .bar > i{display:block;height:100%;background:#fff;border-radius:999px;transition:width .4s}
.hero .barlbl{font-size:.8rem;opacity:.92;margin-top:7px}

.section{margin:0 0 14px;background:var(--card);border-radius:var(--r);box-shadow:var(--sh);overflow:hidden}
.section > h2{
  margin:0;font-size:.82rem;letter-spacing:.6px;text-transform:uppercase;
  padding:13px 16px;color:#fff;background:linear-gradient(135deg,#5c8df6,#6d5efc);
}
.topic{padding:12px 14px;border-top:1px solid var(--line)}
.topic:first-of-type{border-top:none}
.topic h3{margin:0 0 9px;font-size:1rem;display:flex;align-items:center;gap:8px}
.topic h3 .tnum{background:var(--bg);color:var(--brand-d);font-size:.78rem;font-weight:800;
  border-radius:7px;padding:2px 7px;flex:none}
.qrow{display:flex;flex-wrap:wrap;gap:8px}
.qchip{
  position:relative;display:inline-flex;align-items:center;justify-content:center;
  min-width:42px;height:42px;padding:0 6px;border-radius:12px;
  background:var(--bg);color:var(--ink);font-weight:800;font-size:.95rem;
  text-decoration:none;border:2px solid transparent;cursor:pointer;
  transition:transform .08s,border-color .15s,background .15s;
}
.qchip:active{transform:scale(.93)}
.qchip.star{background:#fff5e9;color:var(--accent-d);border-color:#ffd9b3}
.qchip.done{background:var(--good-bg);color:var(--good);border-color:#bfe8d2}
.qchip .dot{position:absolute;top:-5px;right:-5px;font-size:.7rem}
.legend{display:flex;gap:14px;flex-wrap:wrap;color:var(--muted);font-size:.8rem;margin:2px 2px 18px;padding:0 2px}
.legend span{display:inline-flex;align-items:center;gap:5px}
.swatch{width:13px;height:13px;border-radius:4px;display:inline-block}

/* ---------- question page ---------- */
.crumb{color:var(--muted);font-size:.82rem;margin:6px 2px 6px;font-weight:600}
.crumb b{color:var(--brand-d)}
.qcard{background:var(--card);border-radius:var(--r);box-shadow:var(--sh);padding:18px 18px 20px;margin-bottom:14px}
.qhead{display:flex;align-items:center;gap:10px;flex-wrap:wrap;margin-bottom:10px}
.qpos{font-size:.95rem;font-weight:800;color:var(--brand-d)}
.qtag{display:inline-block;font-size:.72rem;font-weight:800;letter-spacing:.5px;text-transform:uppercase;
  padding:3px 9px;border-radius:999px}
.qtag.written{background:#eef0ff;color:var(--brand-d)}
.qtag.mc{background:#fff1e6;color:var(--accent-d)}
.qtext{font-size:1.18rem;font-weight:600;margin:0}
.diagram{margin:16px 0 4px;background:#fbfbff;border:1px solid var(--line);border-radius:14px;padding:10px}
.diagram svg{display:block;width:100%;height:auto;max-height:260px}

/* MC choices */
.choices{list-style:none;margin:16px 0 0;padding:0;display:grid;gap:9px}
.choice{
  display:flex;align-items:center;gap:11px;width:100%;text-align:left;
  background:var(--bg);border:2px solid transparent;border-radius:13px;
  padding:12px 13px;font-size:1rem;cursor:pointer;color:var(--ink);font-family:inherit;
}
.choice:active{transform:scale(.99)}
.choice .key{flex:none;width:28px;height:28px;border-radius:8px;background:#fff;
  display:grid;place-items:center;font-weight:800;color:var(--brand-d);box-shadow:0 1px 3px rgba(0,0,0,.08)}
.choice.sel{border-color:var(--brand);background:#fff}
.choice.correct{border-color:var(--good);background:var(--good-bg)}
.choice.correct .key{background:var(--good);color:#fff}
.choice.wrong{border-color:var(--bad);background:var(--bad-bg)}
.choice.wrong .key{background:var(--bad);color:#fff}
.mcverdict{margin-top:12px;padding:11px 14px;border-radius:12px;font-weight:800;animation:pop .25s ease}
.mcverdict.good{background:var(--good-bg);color:var(--good);border:1px solid #bfe8d2}
.mcverdict.bad{background:var(--bad-bg);color:var(--bad);border:1px solid #f5c2c4}
.mcverdict b{text-decoration:underline}

/* reveal buttons + panels */
.actions{display:flex;flex-wrap:wrap;gap:10px;margin-top:16px}
.btn{
  border:none;border-radius:13px;padding:13px 16px;font-size:1rem;font-weight:800;
  cursor:pointer;font-family:inherit;flex:1 1 auto;min-width:140px;transition:transform .08s,filter .15s;
}
.btn:active{transform:translateY(1px) scale(.99)}
.btn.primary{background:var(--brand);color:#fff;box-shadow:var(--sh)}
.btn.hint{background:var(--hint-bg);color:var(--hint)}
.btn.ghost{background:#fff;color:var(--brand-d);box-shadow:var(--sh)}
.btn:disabled{opacity:.5;cursor:default}

.panel{margin-top:14px;border-radius:14px;padding:14px 16px;animation:pop .25s ease}
@keyframes pop{from{opacity:0;transform:translateY(6px)}to{opacity:1;transform:none}}
.panel.hint{background:var(--hint-bg);border:1px solid #b8e6e0}
.panel.hint h4{color:var(--hint)}
.panel.how{background:#fff;box-shadow:var(--sh)}
.panel.answer{background:var(--good-bg);border:1px solid #bfe8d2}
.panel.answer h4{color:var(--good)}
.panel.keys{background:#f7f7ff;border:1px solid var(--line)}
.panel h4{margin:0 0 8px;font-size:.95rem;display:flex;align-items:center;gap:7px}
.panel p{margin:0 0 4px}
.approach{font-size:1.02rem}

.steps{list-style:none;margin:12px 0 0;padding:0;counter-reset:s;display:grid;gap:10px}
.step{display:flex;gap:12px;background:var(--bg);border-radius:12px;padding:11px 13px;animation:pop .25s ease}
.step .n{counter-increment:s;flex:none;width:27px;height:27px;border-radius:50%;background:var(--brand);
  color:#fff;font-weight:800;display:grid;place-items:center;font-size:.9rem}
.step .n::before{content:counter(s)}
.step .body strong{color:var(--brand-d)}
.step .stitle{font-weight:800;display:block;margin-bottom:1px}
.nextstep{margin-top:12px}

.answer .val{font-size:1.12rem;font-weight:800}
.keys ul{margin:6px 0 0;padding-left:20px}
.keys li{margin:4px 0}

details.worked{margin-top:12px;background:#fff;border:1px solid var(--line);border-radius:14px;overflow:hidden}
details.worked summary{padding:13px 16px;font-weight:800;color:var(--brand-d);cursor:pointer;list-style:none}
details.worked summary::-webkit-details-marker{display:none}
details.worked summary::after{content:"▸";float:right;transition:transform .2s}
details.worked[open] summary::after{transform:rotate(90deg)}
.worked .wbody{padding:0 16px 14px;border-top:1px solid var(--line)}
.worked .wstep{margin:10px 0}
.worked .wstep b{color:var(--ink)}

.gotit{display:flex;align-items:center;justify-content:center;gap:9px;margin-top:14px}
.gotit button{border:2px solid var(--good);background:#fff;color:var(--good);font-weight:800;
  border-radius:999px;padding:9px 18px;cursor:pointer;font-family:inherit;font-size:.95rem}
.gotit button.on{background:var(--good);color:#fff}

/* prev / next */
.nav{display:flex;gap:10px;margin-top:18px}
.nav a{flex:1;text-decoration:none;text-align:center;background:#fff;color:var(--brand-d);
  font-weight:800;border-radius:13px;padding:13px;box-shadow:var(--sh)}
.nav a.disabled{opacity:.4;pointer-events:none}
.nav a small{display:block;font-weight:600;color:var(--muted);font-size:.72rem}

.notice{background:#fff8e8;border:1px solid #ffe2a8;color:#8a6d1f;border-radius:12px;
  padding:11px 14px;font-size:.9rem;margin-top:14px}