:root {
  --bg: #ffffff;
  --surface: #ffffff;
  --ink: #1a1a1a;
  --ink-soft: #5a5a5a;
  --ink-faint: #8a8a8a;
  --border: rgba(0,0,0,0.10);
  --border-strong: rgba(0,0,0,0.18);
  --accent: #2a2a2a;
  --good: #16855a;
  --good-bg: #e9f6ee;
  --good-border: #bfe5cd;
  --bad: #b3261e;
  --bad-bg: #fdecea;
  --bad-border: #f4c1bd;
  --warn-bg: #fff7e0;
  --warn-border: #ead08a;
  --hint: #4d4d4d;
  --pill-bg: #f4f4f4;
  --shadow: 0 1px 0 rgba(0,0,0,0.02);
  --radius: 10px;
  --radius-sm: 6px;
  --mono: ui-monospace, SFMono-Regular, Menlo, Consolas, monospace;
  --sans: -apple-system, BlinkMacSystemFont, "SF Pro Text", system-ui, "Segoe UI", Roboto, Helvetica, Arial, sans-serif;
}

* { box-sizing: border-box; }
html, body { margin: 0; padding: 0; background: var(--bg); color: var(--ink); font-family: var(--sans); -webkit-font-smoothing: antialiased; }
body { min-height: 100vh; }

#app { max-width: 880px; margin: 0 auto; padding: 16px 18px 80px; }

.topbar { display: flex; align-items: center; justify-content: space-between; gap: 12px; padding: 8px 0 18px; border-bottom: 0.5px solid var(--border); }
.brand { font-weight: 600; font-size: 16px; letter-spacing: -0.01em; cursor: pointer; }
.meta { display: flex; align-items: center; gap: 10px; font-size: 12px; color: var(--ink-soft); }
.exam-badge { padding: 4px 8px; border: 0.5px solid var(--border-strong); border-radius: 999px; background: var(--pill-bg); }

.ghost-btn { background: transparent; border: 0.5px solid var(--border-strong); color: var(--ink-soft); font-size: 12px; padding: 4px 10px; border-radius: 999px; cursor: pointer; font-family: var(--sans); }
.ghost-btn:hover { background: var(--pill-bg); }

.foot { margin-top: 30px; padding-top: 14px; border-top: 0.5px solid var(--border); color: var(--ink-faint); font-size: 11px; text-align: center; }

/* DASHBOARD */
.dash-intro { padding: 22px 4px 18px; }
.dash-intro h1 { margin: 0 0 6px; font-size: 22px; font-weight: 600; letter-spacing: -0.02em; }
.dash-intro p { margin: 0; color: var(--ink-soft); font-size: 13px; }
.mode-grid { display: grid; grid-template-columns: 1fr 1fr; gap: 10px; margin-top: 14px; }
@media (max-width: 600px) { .mode-grid { grid-template-columns: 1fr; } }
.mode-card { text-align: left; background: var(--surface); border: 0.5px solid var(--border-strong); border-radius: var(--radius); padding: 14px 14px 12px; cursor: pointer; font-family: var(--sans); transition: border-color .12s ease, transform .08s ease; box-shadow: var(--shadow); }
.mode-card:hover { border-color: var(--ink); }
.mode-card:active { transform: translateY(1px); }
.mode-card .mc-top { display: flex; align-items: center; justify-content: space-between; gap: 8px; }
.mode-card .mc-q { font-size: 11px; color: var(--ink-faint); letter-spacing: 0.06em; text-transform: uppercase; }
.mode-card .mc-stars { font-size: 11px; color: var(--ink-soft); letter-spacing: 0.04em; }
.mode-card h3 { margin: 6px 0 4px; font-size: 15px; font-weight: 600; letter-spacing: -0.01em; }
.mode-card p { margin: 0; color: var(--ink-soft); font-size: 12px; line-height: 1.45; }
.mode-card .mc-stats { display: flex; align-items: center; gap: 8px; margin-top: 10px; font-size: 11px; color: var(--ink-faint); }
.bar { flex: 1; height: 4px; background: #efefef; border-radius: 999px; overflow: hidden; }
.bar > span { display: block; height: 100%; background: var(--ink); transition: width .25s ease; }

/* MODE HEADER */
.mode-head { display: flex; align-items: center; justify-content: space-between; gap: 10px; padding: 16px 0 10px; }
.mode-head .left { min-width: 0; }
.mode-head .left .crumb { font-size: 11px; color: var(--ink-faint); letter-spacing: 0.06em; text-transform: uppercase; }
.mode-head .left h2 { margin: 2px 0 2px; font-size: 18px; font-weight: 600; letter-spacing: -0.01em; }
.mode-head .left .sub { font-size: 12px; color: var(--ink-soft); }
.mode-head .right { display: flex; align-items: center; gap: 8px; font-size: 12px; color: var(--ink-soft); }
.score-pill { padding: 4px 10px; border: 0.5px solid var(--border-strong); border-radius: 999px; background: var(--pill-bg); font-variant-numeric: tabular-nums; }

/* PROGRESS DOTS */
.dots { display: flex; align-items: center; justify-content: center; gap: 8px; padding: 10px 0 14px; }
.dot { width: 10px; height: 10px; border-radius: 50%; border: 0.5px solid var(--border-strong); background: #fff; transition: background .15s ease, border-color .15s ease, transform .12s ease; }
.dot.done { background: var(--ink); border-color: var(--ink); }
.dot.active { transform: scale(1.18); border-color: var(--ink); }
.dot.wrong { background: var(--bad); border-color: var(--bad); }

/* CARD / STEP */
.card { background: var(--surface); border: 0.5px solid var(--border-strong); border-radius: var(--radius); padding: 18px 18px 16px; box-shadow: var(--shadow); }
.step-label { font-size: 11px; color: var(--ink-faint); letter-spacing: 0.06em; text-transform: uppercase; margin-bottom: 6px; }
.step-prompt { font-size: 15px; line-height: 1.55; color: var(--ink); margin: 0 0 14px; }
.problem-stmt { font-size: 14px; color: var(--ink-soft); margin: 0 0 10px; }
.expr-box { background: #fafafa; border: 0.5px solid var(--border); border-radius: var(--radius-sm); padding: 12px 14px; margin: 6px 0 14px; font-size: 16px; }

/* MCQ GRID */
.choices { display: grid; grid-template-columns: 1fr 1fr; gap: 10px; }
@media (max-width: 520px) { .choices { grid-template-columns: 1fr; } }
.choice { background: #fff; border: 0.5px solid var(--border-strong); border-radius: var(--radius-sm); padding: 12px 14px; cursor: pointer; text-align: left; font-family: var(--sans); font-size: 14px; color: var(--ink); transition: border-color .12s ease, background .12s ease; min-height: 48px; display: flex; align-items: center; }
.choice:hover { border-color: var(--ink); }
.choice.correct { background: var(--good-bg); border-color: var(--good-border); color: var(--good); }
.choice.wrong { background: var(--bad-bg); border-color: var(--bad-border); color: var(--bad); }
.choice[disabled] { cursor: default; }

/* FEEDBACK */
.feedback { margin-top: 12px; padding: 10px 12px; border-radius: var(--radius-sm); font-size: 13px; line-height: 1.5; border: 0.5px solid transparent; }
.feedback.good { background: var(--good-bg); border-color: var(--good-border); color: var(--good); }
.feedback.bad { background: var(--bad-bg); border-color: var(--bad-border); color: var(--bad); }
.feedback.hint { background: var(--warn-bg); border-color: var(--warn-border); color: #6b5400; }

/* FINAL ANSWER BANNER */
.final-banner { margin-top: 14px; padding: 14px 16px; border: 0.5px solid var(--ink); border-radius: var(--radius); background: #fafafa; }
.final-banner .label { font-size: 11px; letter-spacing: 0.08em; text-transform: uppercase; color: var(--ink-faint); margin-bottom: 4px; }
.final-banner .ans { font-size: 17px; }
.final-banner .verify { margin-top: 8px; font-size: 12px; color: var(--ink-soft); }

/* BUTTON ROW */
.row { display: flex; gap: 10px; margin-top: 14px; flex-wrap: wrap; }
.btn { background: var(--ink); color: #fff; border: 0; border-radius: var(--radius-sm); padding: 10px 14px; font-size: 13px; font-family: var(--sans); cursor: pointer; }
.btn:hover { background: #000; }
.btn.alt { background: #fff; color: var(--ink); border: 0.5px solid var(--border-strong); }
.btn.alt:hover { background: var(--pill-bg); }

/* SVG / GRAPH */
.graph-wrap { display: flex; justify-content: center; padding: 8px 0 14px; }
svg.graph { background: #fff; border: 0.5px solid var(--border); border-radius: var(--radius-sm); max-width: 100%; height: auto; }
svg.graph .axis { stroke: #888; stroke-width: 0.5; }
svg.graph .grid { stroke: #eee; stroke-width: 0.5; }
svg.graph .tick { stroke: #666; stroke-width: 0.5; }
svg.graph .ticklbl { font: 10px var(--mono); fill: #555; }
svg.graph .curve { fill: none; stroke: #1a1a1a; stroke-width: 1.6; }
svg.graph .curve.dashed { stroke-dasharray: 4 3; }
svg.graph .va { stroke: #b3261e; stroke-width: 0.75; stroke-dasharray: 3 3; }
svg.graph .ha { stroke: #1a73e8; stroke-width: 0.75; stroke-dasharray: 3 3; }
svg.graph circle.open { fill: #fff; stroke: #1a1a1a; stroke-width: 1.4; }
svg.graph circle.closed { fill: #1a1a1a; }

/* SIGN CHART */
.sign-chart { margin: 8px 0 0; }
.sign-row { display: grid; gap: 4px; align-items: stretch; margin-bottom: 10px; }
.sign-row .axis-line { position: relative; height: 32px; border-bottom: 0.5px solid var(--border-strong); }
.sign-row .axis-mark { position: absolute; top: 0; transform: translateX(-50%); display: flex; flex-direction: column; align-items: center; }
.sign-row .axis-mark .tick { width: 1px; height: 8px; background: var(--ink); }
.sign-row .axis-mark .lbl { font: 11px var(--mono); color: var(--ink); margin-top: 2px; }
.sign-cells { display: grid; gap: 6px; margin-top: 8px; }
.sign-cell { display: flex; align-items: center; justify-content: space-between; gap: 8px; padding: 8px 10px; border: 0.5px solid var(--border-strong); border-radius: var(--radius-sm); background: #fff; font-size: 13px; }
.sign-cell .interval { font-family: var(--mono); color: var(--ink-soft); }
.sign-cell .picks { display: flex; gap: 6px; }
.sign-cell button { background: #fff; border: 0.5px solid var(--border-strong); padding: 4px 12px; border-radius: 999px; cursor: pointer; font-family: var(--mono); font-size: 14px; min-width: 38px; }
.sign-cell button.picked-pos { background: var(--good-bg); border-color: var(--good-border); color: var(--good); }
.sign-cell button.picked-neg { background: var(--bad-bg); border-color: var(--bad-border); color: var(--bad); }

/* RIEMANN canvas */
.riemann-wrap { display: flex; justify-content: center; padding: 4px 0 12px; }
canvas.riemann { background: #fff; border: 0.5px solid var(--border); border-radius: var(--radius-sm); max-width: 100%; height: auto; }
.slider-row { display: flex; align-items: center; gap: 12px; margin: 8px 0; font-size: 13px; color: var(--ink-soft); }
.slider-row input[type=range] { flex: 1; }

/* COMPLETED */
.complete-card { text-align: center; padding: 26px 18px; }
.complete-card h2 { margin: 4px 0 6px; }
.complete-card p { margin: 0 0 14px; color: var(--ink-soft); font-size: 13px; }

/* INPUT */
.input-row { display: flex; gap: 8px; align-items: center; margin-top: 8px; }
.input-row input { flex: 1; padding: 10px 12px; border: 0.5px solid var(--border-strong); border-radius: var(--radius-sm); font-family: var(--mono); font-size: 14px; }
.input-row input:focus { outline: 1px solid var(--ink); border-color: var(--ink); }

/* FOCUS RING — keyboard a11y */
.btn:focus-visible, .choice:focus-visible, .mode-card:focus-visible,
.sign-cell button:focus-visible, .home-link:focus-visible, .ghost-btn:focus-visible {
  outline: 2px solid var(--ink); outline-offset: 2px;
}

/* METHOD PANEL */
.method-card { border: 0.5px solid var(--border-strong); border-radius: var(--radius); background: #fafafa; margin-bottom: 12px; overflow: hidden; }
.method-card.open { background: #fbfbf7; }
.method-head { width: 100%; text-align: left; background: transparent; border: none; padding: 10px 14px; font-family: var(--sans); font-size: 13px; color: var(--ink); cursor: pointer; display: flex; align-items: center; gap: 8px; }
.method-head:hover { background: rgba(0,0,0,0.02); }
.method-caret { font-size: 11px; color: var(--ink-soft); width: 12px; display: inline-block; }
.method-title { font-weight: 600; letter-spacing: -0.005em; }
.method-body { padding: 4px 18px 14px 34px; font-size: 13px; line-height: 1.55; color: var(--hint); border-top: 0.5px solid var(--border); }
.method-body p { margin: 8px 0; }
.method-body ol, .method-body ul { margin: 6px 0; padding-left: 20px; }
.method-body li { margin: 4px 0; }
.method-body b { color: var(--ink); font-weight: 600; }
.method-body .katex { font-size: 1em; }
.method-hint { color: var(--ink-soft); font-weight: 400; font-style: italic; margin-left: auto; font-size: 12px; }
.method-intro { padding: 8px 12px; background: #fff; border: 0.5px solid var(--border); border-radius: 6px; margin: 8px 0 12px; color: var(--ink); }
.method-step { margin: 10px 0; padding: 8px 12px; background: #fff; border-left: 2px solid var(--ink); border-radius: 0 6px 6px 0; }
.method-step-label { font-weight: 600; color: var(--ink); font-size: 12px; text-transform: uppercase; letter-spacing: 0.04em; margin-bottom: 4px; }
.method-step-body { color: var(--ink); }
.method-controls { display: flex; gap: 8px; margin-top: 12px; flex-wrap: wrap; align-items: center; }
.method-final { padding: 10px 12px; background: #f3f7f0; border: 0.5px solid #c9d6bc; border-radius: 6px; color: var(--ink); width: 100%; margin-bottom: 8px; }