:root{
  --bg:#f7f7fb;
  --card:#ffffff;
  --ink:#1f2937;
  --muted:#6b7280;
  --blue:#2563eb;
  --green:#16a34a;
  --orange:#f97316;
  --purple:#7c3aed;
  --red:#dc2626;
  --line:#d1d5db;
  --soft:#eef2ff;
}
*{box-sizing:border-box}
body{margin:0;font-family:system-ui,-apple-system,BlinkMacSystemFont,"Segoe UI",sans-serif;background:var(--bg);color:var(--ink)}
.app{width:100%;min-height:100vh;padding:14px;display:flex;flex-direction:column;gap:12px}
header{background:linear-gradient(135deg,#eef2ff,#ffffff);border:1px solid var(--line);border-radius:18px;padding:14px 16px}
h1{margin:0 0 6px;font-size:clamp(20px,3vw,30px)}
.problem{margin:0;color:var(--muted);line-height:1.45;font-size:15px}
.main{display:grid;grid-template-columns:minmax(320px,1.15fr) minmax(320px,0.85fr);gap:12px;align-items:start}
@media(max-width:850px){.main{grid-template-columns:1fr}.app{padding:10px}.card{padding:10px}canvas{min-height:340px;height:52vh}.math{font-size:17px}.eq{font-size:18px}.frac{font-size:0.95em}}
.card{background:var(--card);border:1px solid var(--line);border-radius:18px;padding:14px;box-shadow:0 8px 20px rgba(15,23,42,0.06)}
.canvas-wrap{position:relative;width:100%}
canvas{width:100%;height:min(62vh,560px);min-height:390px;border-radius:14px;background:#fff;border:1px solid var(--line);touch-action:none}
.legend{display:flex;flex-wrap:wrap;gap:8px;margin-top:10px;font-size:13px}
.tag{display:inline-flex;align-items:center;gap:6px;padding:6px 9px;border-radius:999px;background:#f3f4f6;border:1px solid #e5e7eb}
.dot{width:10px;height:10px;border-radius:50%;display:inline-block}.blue{background:var(--blue)}.green{background:var(--green)}.orange{background:var(--orange)}.purple{background:var(--purple)}
.controls{display:flex;flex-wrap:wrap;gap:8px;margin-top:10px}
button{min-height:44px;padding:10px 13px;border:none;border-radius:12px;background:var(--blue);color:white;font-weight:700;cursor:pointer;font-size:14px}
button.secondary{background:#e5e7eb;color:#111827}button.ghost{background:#fff;color:#111827;border:1px solid var(--line)}button:hover{filter:brightness(0.97)}
.step-title{display:flex;align-items:flex-start;justify-content:space-between;gap:10px;margin-bottom:10px}h2{margin:0;font-size:clamp(22px,4vw,34px);line-height:1.12}.pill{padding:5px 10px;border-radius:999px;background:var(--soft);color:#3730a3;font-weight:700;font-size:13px;white-space:nowrap;margin-top:4px}
.math{background:#f9fafb;border:1px solid #e5e7eb;border-radius:14px;padding:12px;line-height:1.75;font-size:18px;overflow-x:auto}.math .small{font-size:14px;color:var(--muted);line-height:1.5;margin-top:8px}.work{display:grid;gap:7px}.eq{font-family:"Cambria Math","STIX Two Math","Times New Roman",serif;font-size:clamp(18px,3.7vw,28px);line-height:1.45;white-space:normal}.mathline{font-family:"Cambria Math","STIX Two Math","Times New Roman",serif}
.frac{display:inline-grid;grid-template-rows:auto auto;vertical-align:middle;text-align:center;line-height:1.15;margin:0 0.15em;font-family:"Cambria Math","STIX Two Math","Times New Roman",serif}.frac .top{border-bottom:1.6px solid currentColor;padding:0 0.18em 0.06em}.frac .bottom{padding:0.06em 0.18em 0}
.highlight{background:#fff7ed;border-left:5px solid var(--orange);padding:10px 12px;border-radius:12px;margin-top:10px;line-height:1.45;font-size:16px}.foundation{display:grid;gap:8px;margin-top:12px}details{border:1px solid #e5e7eb;border-radius:12px;padding:10px 12px;background:#ffffff}summary{cursor:pointer;font-weight:700;font-size:17px}.mini{color:var(--muted);font-size:14px;line-height:1.45;margin-top:7px}.quiz{margin-top:12px;display:grid;gap:8px}.quiz-row{display:grid;grid-template-columns:1fr auto;gap:8px;align-items:center}input{min-height:44px;padding:9px 11px;border:1px solid var(--line);border-radius:12px;font-size:15px}.feedback{min-height:24px;font-weight:700;font-size:14px}.good{color:var(--green)}.bad{color:var(--red)}.slider-box{margin-top:12px;padding:10px;border-radius:14px;background:#f9fafb;border:1px solid #e5e7eb}input[type="range"]{width:100%;min-height:32px}.answer-box{margin-top:12px;background:#ecfdf5;border:1px solid #bbf7d0;border-radius:14px;padding:12px;line-height:1.55}