/* ============================================================
   微分探索之旅 — 共用樣式
   ============================================================ */
:root {
  --bg: #f6f8fb;
  --panel: #ffffff;
  --ink: #1e293b;
  --ink-soft: #475569;
  --ink-faint: #94a3b8;
  --line: #e2e8f0;

  /* 全站統一的數學物件配色 */
  --c-f: #2563eb;        /* 函數曲線 f  藍 */
  --c-secant: #f59e0b;   /* 割線 橘 */
  --c-tangent: #059669;  /* 切線 綠 */
  --c-deriv: #8b5cf6;    /* 導函數 f' 紫 */
  --c-normal: #ec4899;   /* 法線 粉 */
  --c-accel: #dc2626;    /* 加速度 / 第三曲線 紅 */
  --c-point: #0f172a;

  /* 四個部分的主題色 */
  --part1: #0ea5e9;
  --part2: #8b5cf6;
  --part3: #f59e0b;
  --part4: #10b981;

  --accent: var(--part1);
  --accent-soft: color-mix(in srgb, var(--accent) 12%, white);

  --radius: 14px;
  --shadow: 0 1px 3px rgba(15, 23, 42, .06), 0 8px 24px rgba(15, 23, 42, .05);
  --sans: "PingFang TC", "Microsoft JhengHei", "Noto Sans TC", -apple-system, "Segoe UI", sans-serif;
  --math: Georgia, "Times New Roman", serif;
}

* { box-sizing: border-box; }
html { -webkit-text-size-adjust: 100%; }
body {
  margin: 0;
  font-family: var(--sans);
  background: var(--bg);
  color: var(--ink);
  line-height: 1.75;
  font-size: 16px;
}

/* ---------- 頁首 ---------- */
.site-header {
  position: sticky; top: 0; z-index: 50;
  background: rgba(255,255,255,.92);
  backdrop-filter: blur(8px);
  border-bottom: 1px solid var(--line);
  display: flex; align-items: center; justify-content: space-between;
  padding: 10px 20px; gap: 12px;
}
.site-header .home {
  font-weight: 700; text-decoration: none; color: var(--ink);
  display: flex; align-items: center; gap: 8px; font-size: 16px;
  white-space: nowrap;
}
.site-header .home .logo {
  width: 30px; height: 30px; border-radius: 8px;
  background: linear-gradient(135deg, #2563eb, #8b5cf6);
  color: #fff; display: grid; place-items: center;
  font-family: var(--math); font-style: italic; font-size: 17px;
}
.site-header nav { display: flex; gap: 8px; }
.site-header nav a {
  text-decoration: none; color: var(--ink-soft);
  border: 1px solid var(--line); border-radius: 999px;
  padding: 5px 14px; font-size: 14px; background: #fff;
  white-space: nowrap; transition: .15s;
}
.site-header nav a:hover { border-color: var(--accent); color: var(--accent); }
.site-header nav a.disabled { opacity: .35; pointer-events: none; }

main { max-width: 1060px; margin: 0 auto; padding: 28px 20px 80px; }

/* ---------- 課程標題 ---------- */
.lesson-head { margin: 6px 0 20px; }
.lesson-head .chip {
  display: inline-block; font-size: 13px; font-weight: 600;
  color: var(--accent);
  background: var(--accent-soft);
  border-radius: 999px; padding: 3px 12px; margin-bottom: 10px;
}
.lesson-head h1 { margin: 0; font-size: 30px; line-height: 1.3; letter-spacing: .01em; }
.lesson-head h1 .en {
  display: block; font-size: 16px; font-weight: 500;
  color: var(--ink-faint); font-family: var(--math); font-style: italic;
  margin-top: 2px; letter-spacing: .02em;
}

/* ---------- 區塊 ---------- */
.panel {
  background: var(--panel); border: 1px solid var(--line);
  border-radius: var(--radius); box-shadow: var(--shadow);
  padding: 22px 24px; margin: 18px 0;
}
.panel h2 {
  margin: 0 0 10px; font-size: 20px;
  display: flex; align-items: center; gap: 10px;
}
.panel h2::before {
  content: ""; width: 5px; height: 20px; border-radius: 3px;
  background: var(--accent); flex: none;
}
.panel h3 { font-size: 16px; margin: 18px 0 8px; }
.panel p { margin: 8px 0; color: var(--ink-soft); }
.panel p strong, .panel li strong { color: var(--ink); }
.term { font-weight: 700; color: var(--ink); }
.term .en { font-family: var(--math); font-style: italic; font-weight: 500; color: var(--ink-faint); font-size: .92em; }

/* ---------- 互動示範區 ---------- */
.demo-grid { display: grid; grid-template-columns: 3fr 2fr; gap: 20px; align-items: start; }
.demo-grid.wide-right { grid-template-columns: 1fr 1fr; }
.demo-stack { display: flex; flex-direction: column; gap: 12px; }
@media (max-width: 860px) { .demo-grid, .demo-grid.wide-right { grid-template-columns: 1fr; } }

.canvas-wrap { position: relative; }
canvas.plot {
  width: 100%; display: block;
  border: 1px solid var(--line); border-radius: 10px;
  background: #fdfefe; touch-action: none;
}
.canvas-caption { font-size: 13px; color: var(--ink-faint); margin-top: 6px; text-align: center; }

/* ---------- 控制列 ---------- */
.controls { display: flex; flex-direction: column; gap: 14px; }
.control-row { display: flex; align-items: center; gap: 10px; flex-wrap: wrap; }

.ctl-slider { display: flex; flex-direction: column; gap: 4px; width: 100%; }
.ctl-slider .lab {
  display: flex; justify-content: space-between; font-size: 14px; color: var(--ink-soft);
}
.ctl-slider .lab .val {
  font-family: var(--math); font-weight: 700; color: var(--ink);
  background: var(--accent-soft); border-radius: 6px; padding: 0 8px; min-width: 52px; text-align: center;
}
input[type=range] {
  width: 100%; accent-color: var(--accent); height: 26px; margin: 0;
}

button.btn {
  font-family: var(--sans); font-size: 14px; font-weight: 600;
  border: 1px solid var(--line); background: #fff; color: var(--ink-soft);
  border-radius: 10px; padding: 8px 16px; cursor: pointer; transition: .15s;
}
button.btn:hover { border-color: var(--accent); color: var(--accent); }
button.btn.primary {
  background: var(--accent); border-color: var(--accent); color: #fff;
}
button.btn.primary:hover { filter: brightness(1.08); }

.seg { display: inline-flex; background: #eef2f7; border-radius: 10px; padding: 3px; gap: 2px; flex-wrap: wrap; }
.seg button {
  font-family: var(--sans); font-size: 14px; border: none; background: transparent;
  padding: 6px 13px; border-radius: 8px; cursor: pointer; color: var(--ink-soft);
  transition: .15s; white-space: nowrap;
}
.seg button.on { background: #fff; color: var(--ink); font-weight: 700; box-shadow: 0 1px 3px rgba(15,23,42,.12); }
.seg button i { font-family: var(--math); font-style: italic; }

/* ---------- 數值顯示 ---------- */
.readout {
  background: #f8fafc; border: 1px solid var(--line); border-radius: 10px;
  padding: 12px 16px; font-size: 15px;
}
.readout .big { font-size: 20px; font-family: var(--math); }
.readout-row { display: flex; justify-content: space-between; gap: 10px; padding: 3px 0; }
.readout-row .k { color: var(--ink-soft); }
.readout-row .v { font-family: var(--math); font-weight: 700; }

table.num-table {
  border-collapse: collapse; width: 100%; font-size: 14px;
  font-variant-numeric: tabular-nums;
}
table.num-table th, table.num-table td {
  border: 1px solid var(--line); padding: 5px 10px; text-align: center;
  font-family: var(--math);
}
table.num-table th { background: #f1f5f9; font-family: var(--sans); font-size: 13px; }
table.num-table tr.hl td { background: var(--accent-soft); font-weight: 700; }

/* ---------- 數學式排版 ---------- */
.math {
  font-family: var(--math); font-style: italic; white-space: nowrap;
}
.math b, .math .up { font-style: normal; }
.math sup { font-size: .72em; }
.math sub { font-size: .72em; font-style: normal; }
.formula {
  font-family: var(--math); font-size: 19px; text-align: center;
  padding: 14px 10px; margin: 10px 0;
  background: #f8fafc; border: 1px solid var(--line); border-radius: 10px;
  overflow-x: auto;
}
.formula .in { display: inline-flex; align-items: center; gap: 7px; font-style: italic; }
.frac { display: inline-flex; flex-direction: column; vertical-align: middle; text-align: center; margin: 0 2px; }
.frac > .num { padding: 0 5px 1px; border-bottom: 1.4px solid currentColor; }
.frac > .den { padding: 1px 5px 0; }
.lim { display: inline-flex; flex-direction: column; align-items: center; line-height: 1.05; vertical-align: middle; }
.lim .l { font-style: normal; }
.lim .sub { font-size: .7em; font-style: italic; }
.c-f { color: var(--c-f); }
.c-secant { color: var(--c-secant); }
.c-tangent { color: var(--c-tangent); }
.c-deriv { color: var(--c-deriv); }
.c-normal { color: var(--c-normal); }
.c-accel { color: var(--c-accel); }

/* ---------- 重點整理 ---------- */
.keypoints {
  border-left: 5px solid var(--accent);
}
.keypoints ul { margin: 6px 0; padding-left: 22px; }
.keypoints li { margin: 7px 0; color: var(--ink-soft); }

/* ---------- 試一試 ---------- */
.quiz-q { margin: 16px 0 22px; }
.quiz-q .q-text { font-weight: 600; margin-bottom: 10px; }
.quiz-opts { display: flex; flex-direction: column; gap: 8px; }
.quiz-opts button {
  text-align: left; font-family: var(--sans); font-size: 15px;
  border: 1.5px solid var(--line); background: #fff; border-radius: 10px;
  padding: 10px 14px; cursor: pointer; transition: .15s; color: var(--ink);
  line-height: 1.6;
}
.quiz-opts button:hover:not(:disabled) { border-color: var(--accent); }
.quiz-opts button.correct { border-color: #059669; background: #ecfdf5; }
.quiz-opts button.wrong { border-color: #dc2626; background: #fef2f2; }
.quiz-opts button:disabled { cursor: default; }
.quiz-fb { margin-top: 10px; font-size: 14.5px; border-radius: 10px; padding: 10px 14px; display: none; }
.quiz-fb.ok { display: block; background: #ecfdf5; color: #065f46; }
.quiz-fb.no { display: block; background: #fef2f2; color: #991b1b; }

/* ---------- 頁尾導覽 ---------- */
.lesson-footer {
  display: flex; justify-content: space-between; align-items: center; gap: 12px;
  margin-top: 34px; flex-wrap: wrap;
}
.lesson-footer a {
  text-decoration: none; color: var(--ink);
  border: 1px solid var(--line); background: #fff; border-radius: 12px;
  padding: 12px 18px; font-size: 15px; box-shadow: var(--shadow);
  transition: .15s; max-width: 46%;
}
.lesson-footer a:hover { border-color: var(--accent); }
.lesson-footer a .dir { display: block; font-size: 12px; color: var(--ink-faint); }
.lesson-footer .spacer { flex: 1; }
.done-btn {
  border: 1.5px solid var(--line); background: #fff; border-radius: 999px;
  font-family: var(--sans); font-size: 14.5px; font-weight: 600; color: var(--ink-soft);
  padding: 9px 20px; cursor: pointer; transition: .15s;
}
.done-btn.on { background: #059669; border-color: #059669; color: #fff; }

/* ---------- 首頁 ---------- */
.hero { text-align: center; padding: 36px 10px 10px; }
.hero h1 { font-size: 36px; margin: 0 0 6px; }
.hero .sub { color: var(--ink-soft); font-size: 17px; max-width: 640px; margin: 0 auto; }
.hero .math-deco { font-family: var(--math); font-style: italic; font-size: 20px; color: var(--c-deriv); }
.progress-wrap { max-width: 420px; margin: 22px auto 8px; }
.progress-bar { height: 10px; background: #e2e8f0; border-radius: 999px; overflow: hidden; }
.progress-bar > div { height: 100%; background: linear-gradient(90deg, #2563eb, #8b5cf6); border-radius: 999px; transition: width .4s; }
.progress-label { font-size: 13px; color: var(--ink-faint); margin-top: 6px; text-align: center; }

.part-block { margin: 34px 0 10px; }
.part-block .part-title { display: flex; align-items: baseline; gap: 10px; margin-bottom: 4px; }
.part-block .part-title .no {
  font-size: 13px; font-weight: 800; letter-spacing: .12em;
  color: var(--accent); text-transform: uppercase;
}
.part-block .part-title h2 { margin: 0; font-size: 22px; }
.part-block .part-desc { color: var(--ink-soft); font-size: 14.5px; margin: 0 0 14px; }
.card-grid { display: grid; grid-template-columns: repeat(auto-fill, minmax(230px, 1fr)); gap: 14px; }
.lesson-card {
  position: relative; display: block; text-decoration: none; color: var(--ink);
  background: var(--panel); border: 1px solid var(--line); border-radius: var(--radius);
  box-shadow: var(--shadow); padding: 16px 18px 14px; transition: .18s;
  border-top: 4px solid var(--accent);
}
.lesson-card:hover { transform: translateY(-3px); box-shadow: 0 6px 20px rgba(15,23,42,.1); }
.lesson-card .no {
  font-family: var(--math); font-size: 13px; color: var(--ink-faint);
}
.lesson-card h3 { margin: 2px 0 2px; font-size: 17.5px; }
.lesson-card .en { font-family: var(--math); font-style: italic; font-size: 13px; color: var(--ink-faint); display: block; margin-bottom: 6px; }
.lesson-card p { margin: 0; font-size: 13.5px; color: var(--ink-soft); line-height: 1.6; }
.lesson-card .tick {
  position: absolute; top: 10px; right: 12px; width: 22px; height: 22px;
  border-radius: 50%; display: none; place-items: center;
  background: #059669; color: #fff; font-size: 13px;
}
.lesson-card.done .tick { display: grid; }

/* ---------- 雜項 ---------- */
.hint-box {
  background: var(--accent-soft); border-radius: 10px; padding: 10px 16px;
  font-size: 14.5px; color: var(--ink-soft); margin: 10px 0;
}
.legend { display: flex; gap: 16px; flex-wrap: wrap; font-size: 13.5px; color: var(--ink-soft); margin: 8px 0 2px; }
.legend .item { display: inline-flex; align-items: center; gap: 6px; }
.legend .sw { width: 18px; height: 4px; border-radius: 2px; display: inline-block; }
.legend .sw.dot { width: 10px; height: 10px; border-radius: 50%; }

.footnote { text-align: center; color: var(--ink-faint); font-size: 13px; margin-top: 40px; }