:root {
  --paper: #eef3f7;
  --grid: #d3dfea;
  --ink: #14213d;
  --muted: #3f5070;
  --panel: #ffffff;
  --path: #2f5bea;
  --mark: #ffd84d;
  --ball: #e5484d;
  --bad: #b3261e;
  color-scheme: light dark;
}

@media (prefers-color-scheme: dark) {
  :root {
    --paper: #0f1a2e;
    --grid: #1b2a44;
    --ink: #eaf0fa;
    --muted: #a9b9d4;
    --panel: #13233d;
    --path: #7ea2ff;
    --mark: #ffd84d;
    --ball: #ff7a7f;
    --bad: #ff8a8f;
  }
}

* { box-sizing: border-box; }

html { -webkit-text-size-adjust: 100%; }

body {
  margin: 0;
  font-family: "Sarabun", "Noto Sans Thai", system-ui, sans-serif;
  font-size: 1.0625rem;
  line-height: 1.7;
  color: var(--ink);
  background-color: var(--paper);
  background-image:
    linear-gradient(var(--grid) 1px, transparent 1px),
    linear-gradient(90deg, var(--grid) 1px, transparent 1px);
  background-size: 28px 28px;
}

h1, h2, .brand {
  font-family: "Prompt", "Sarabun", system-ui, sans-serif;
  font-weight: 600;
  line-height: 1.35;
}

.wrap { max-width: 1080px; margin: 0 auto; padding: 0 20px; }

a { color: inherit; }

:focus-visible { outline: 3px solid var(--path); outline-offset: 2px; }

/* ส่วนหัว */
.site-header { padding: 18px 0; }
.site-header .wrap { display: flex; align-items: center; justify-content: space-between; gap: 16px; }
.site-header nav a { font-weight: 600; padding: 8px 4px; }
.brand {
  display: inline-flex;
  align-items: center;
  gap: 10px;
  font-size: 1.25rem;
  text-decoration: none;
}

/* ส่วนบนสุดของหน้า */
.hero {
  display: grid;
  grid-template-columns: 1fr;
  gap: 28px;
  padding-top: 20px;
  padding-bottom: 44px;
}

@media (min-width: 860px) {
  .hero {
    grid-template-columns: 1fr 1.15fr;
    align-items: center;
    gap: 48px;
    padding-top: 44px;
    padding-bottom: 64px;
  }
}

h1 {
  font-size: clamp(1.85rem, 4.6vw, 2.85rem);
  margin: 0 0 16px;
  max-width: 15em;
  text-wrap: balance;
}

.lead { max-width: 34em; margin: 0 0 18px; color: var(--muted); }

.status {
  display: inline-block;
  margin: 0;
  padding: 2px 12px;
  background: var(--mark);
  color: #14213d;
  font-weight: 600;
}

.more { margin: 14px 0 0; }
.more a { font-weight: 600; }

/* ตัวอย่างจำลอง */
.sim {
  background: var(--panel);
  border: 2px solid var(--ink);
  padding: 16px;
}

.sim h2 { font-size: 1.05rem; margin: 0 0 8px; }

#plot { display: block; width: 100%; height: auto; }

.g-line { stroke: var(--grid); stroke-width: 1; }
.g-axis { stroke: var(--ink); stroke-width: 1.5; }
.g-label { fill: var(--muted); font-size: 20px; font-family: "Sarabun", sans-serif; }
.path { fill: none; stroke: var(--path); stroke-width: 3; stroke-linejoin: round; }
.mark { fill: var(--mark); stroke: var(--ink); stroke-width: 1.5; }
.ball { fill: var(--ball); stroke: var(--ink); stroke-width: 1.5; }

.controls {
  display: grid;
  grid-template-columns: 1fr;
  gap: 10px 20px;
  margin-top: 12px;
}

@media (min-width: 560px) {
  .controls { grid-template-columns: 1fr 1fr; }
  .controls button { grid-column: 1 / -1; }
}

.field label { display: block; font-weight: 600; }
input[type="range"] { width: 100%; accent-color: var(--path); }

button {
  font-family: "Prompt", "Sarabun", system-ui, sans-serif;
  font-size: 1rem;
  font-weight: 600;
  min-height: 44px;
  padding: 8px 22px;
  color: var(--paper);
  background: var(--ink);
  border: 2px solid var(--ink);
  cursor: pointer;
}

button:hover { background: var(--path); border-color: var(--path); color: #fff; }

.results {
  display: grid;
  grid-template-columns: repeat(3, 1fr);
  gap: 8px;
  margin: 14px 0 0;
}

.results dt { font-size: 0.85rem; color: var(--muted); }

.results dd {
  margin: 0;
  font-family: "Prompt", "Sarabun", system-ui, sans-serif;
  font-weight: 600;
  font-size: 1.1rem;
}

.note { margin: 10px 0 0; font-size: 0.9rem; color: var(--muted); }

/* ขั้นตอนของบทเรียน */
.steps {
  background: var(--paper);
  border-top: 2px solid var(--ink);
  padding: 36px 0 52px;
}

.steps h2 { font-size: 1.5rem; margin: 0 0 20px; }

.steps ol {
  list-style: none;
  counter-reset: step;
  display: grid;
  grid-template-columns: 1fr;
  gap: 18px;
  margin: 0;
  padding: 0;
}

@media (min-width: 760px) {
  .steps ol { grid-template-columns: repeat(4, 1fr); gap: 24px; }
}

.steps li {
  counter-increment: step;
  display: flex;
  flex-direction: column;
  gap: 4px;
  padding-top: 12px;
  border-top: 2px solid var(--ink);
}

.steps li::before {
  content: counter(step);
  font-family: "Prompt", "Sarabun", system-ui, sans-serif;
  font-weight: 600;
  font-size: 1.6rem;
  line-height: 1;
  color: var(--path);
}

.steps li strong { font-family: "Prompt", "Sarabun", system-ui, sans-serif; font-weight: 600; }
.steps li span { color: var(--muted); }
.soon { margin: 28px 0 0; color: var(--muted); }

/* หน้าหลักสูตร */
.solid { background: var(--paper); }
.courses { padding-top: 24px; padding-bottom: 64px; }
.courses h1 { font-size: clamp(1.7rem, 4vw, 2.4rem); }
.total { margin: 0 0 20px; font-weight: 600; }
#catalog { display: grid; grid-template-columns: 1fr; gap: 28px; }
#catalog .total { grid-column: 1 / -1; margin: 0; }
@media (min-width: 760px) { #catalog { grid-template-columns: 1fr 1fr; gap: 32px 48px; } }
.cat h2 {
  display: flex;
  justify-content: space-between;
  align-items: baseline;
  gap: 12px;
  font-size: 1.2rem;
  margin: 0 0 8px;
  padding-top: 12px;
  border-top: 2px solid var(--ink);
}
.cat-count { font-family: "Sarabun", sans-serif; font-weight: 400; font-size: 0.95rem; color: var(--muted); white-space: nowrap; }
.chapters { list-style: none; margin: 0; padding: 0; }
.chapters li { padding: 6px 0; border-bottom: 1px solid var(--grid); }
.chapters .row { display: flex; justify-content: space-between; gap: 12px; }
.lesson-links { list-style: none; margin: 4px 0 4px 16px; padding: 0; }
.lesson-links li { border: 0; padding: 2px 0; }
.lesson-links a { font-weight: 600; color: var(--path); }
.ch-count { color: var(--muted); white-space: nowrap; }

/* หน้าบทเรียน */
.lesson { padding-top: 20px; padding-bottom: 64px; }
.lesson h1 { font-size: clamp(1.6rem, 4vw, 2.3rem); margin: 8px 0 8px; max-width: 22em; }
.crumbs { font-size: 0.95rem; color: var(--muted); }
.crumbs a { font-weight: 600; }
.level-badge {
  display: inline-block;
  margin: 0 0 8px;
  padding: 0 10px;
  border: 2px solid var(--ink);
  font-weight: 600;
  font-size: 0.9rem;
}
.step { margin-top: 36px; padding-top: 14px; border-top: 2px solid var(--ink); }
.step h2 { display: flex; align-items: baseline; gap: 12px; flex-wrap: wrap; font-size: 1.3rem; margin: 0 0 14px; }
.step-no { color: var(--path); font-size: 1.7rem; line-height: 1; }
.step-status { margin-left: auto; padding: 0 10px; background: var(--mark); color: #14213d; font-family: "Sarabun", sans-serif; font-size: 0.9rem; font-weight: 600; }
.step-status:empty { display: none; }
.muted { color: var(--muted); }
.videos { list-style: none; margin: 0 0 12px; padding: 0; display: grid; gap: 14px; }
.videos li { display: flex; flex-direction: column; align-items: flex-start; gap: 4px; padding: 12px; background: var(--panel); border: 2px solid var(--ink); }
a.button, button.secondary {
  display: inline-block;
  font-family: "Prompt", "Sarabun", system-ui, sans-serif;
  font-weight: 600;
  text-decoration: none;
  min-height: 44px;
  padding: 8px 22px;
  line-height: 1.7;
  color: var(--paper);
  background: var(--ink);
  border: 2px solid var(--ink);
}
a.button:hover { background: var(--path); border-color: var(--path); color: #fff; }
button.secondary { color: var(--ink); background: var(--panel); }
button.secondary:hover { background: var(--mark); border-color: var(--ink); color: #14213d; }
.field.check { display: flex; align-items: center; gap: 10px; min-height: 44px; }
.field.check input { width: 22px; height: 22px; accent-color: var(--path); }
.controls.three { grid-template-columns: 1fr; }
@media (min-width: 560px) { .controls.three { grid-template-columns: 1fr 1fr; } }
.results.two { grid-template-columns: repeat(2, 1fr); }
.target { fill: var(--mark); stroke: var(--ink); stroke-width: 1.5; }
.ray { stroke: var(--path); stroke-width: 4; stroke-linecap: round; }
.normal { stroke: var(--muted); stroke-width: 2; stroke-dasharray: 8 6; }
.mirror { stroke: var(--ink); stroke-width: 7; stroke-linecap: round; }
.arc { fill: none; stroke: var(--ink); stroke-width: 2; }
.arrow { stroke: var(--ink); stroke-width: 4; stroke-linecap: round; }
.arrow-head { fill: var(--ink); }
.charge-pos { fill: var(--ball); stroke: var(--ink); stroke-width: 2; }
.charge-neg { fill: var(--path); stroke: var(--ink); stroke-width: 2; }
.charge-sign { fill: #fff; font-family: "Prompt", sans-serif; font-weight: 600; font-size: 26px; }
@media (prefers-color-scheme: dark) { .charge-sign { fill: #0f1a2e; } }
.game-info, .game-msg { margin: 0 0 10px; }
.game-msg { margin-top: 10px; font-weight: 600; min-height: 1.7em; }
.quiz .q { margin: 0 0 18px; padding: 12px 14px; background: var(--panel); border: 2px solid var(--ink); }
.quiz legend { float: left; width: 100%; padding: 0 0 8px; font-weight: 600; }
.quiz .opt, .quiz .mark-line { clear: both; }
.opt { display: flex; align-items: center; gap: 10px; min-height: 44px; }
.opt input[type="radio"] { width: 22px; height: 22px; accent-color: var(--path); flex: none; }
.opt.num { flex-direction: column; align-items: flex-start; gap: 4px; }
.opt.num input { font: inherit; width: 12em; max-width: 100%; padding: 8px 10px; color: var(--ink); background: var(--paper); border: 2px solid var(--ink); }
.mark-line { margin: 8px 0 0; }
.mark-line:empty { display: none; }
.mark-line.ok { color: var(--path); }
.mark-line.bad { color: var(--bad); }
.mark-line strong { color: inherit; }
.actions { display: flex; gap: 12px; flex-wrap: wrap; }
.quiz-result { margin-top: 14px; }
.score { margin: 0; font-family: "Prompt", "Sarabun", sans-serif; font-weight: 600; font-size: 1.15rem; }
.save-note { margin-top: 28px; }

/* หน้า 404 */
.notfound { padding-top: 40px; padding-bottom: 80px; }
.back { font-weight: 600; }

/* ส่วนท้าย */
.site-footer {
  background: var(--ink);
  color: var(--paper);
  padding: 24px 0 32px;
  font-size: 0.95rem;
}

.site-footer p { margin: 4px 0; max-width: 46em; }

@media (prefers-reduced-motion: reduce) {
  * { scroll-behavior: auto !important; transition: none !important; animation: none !important; }
}