/* ==========================================================================
   RESISTOR LAB — Design System
   Dark engineering UI. Inter for interface text, JetBrains Mono for values.
   ========================================================================== */

:root {
  /* Palette */
  --bg: #0B1120;
  --card: #111827;
  --surface-2: #1E293B;
  --text-primary: #F8FAFC;
  --text-secondary: #94A3B8;
  --accent-blue: #38BDF8;
  --accent-blue-dim: #0EA5E9;
  --accent-green: #22C55E;
  --warning: #F59E0B;
  --danger: #EF4444;
  --border: rgba(148, 163, 184, 0.14);
  --border-strong: rgba(148, 163, 184, 0.28);
  --shadow: 0 20px 40px -20px rgba(0, 0, 0, 0.6);

  /* Type */
  --font-ui: 'Inter', -apple-system, BlinkMacSystemFont, 'Segoe UI', sans-serif;
  --font-mono: 'JetBrains Mono', 'SFMono-Regular', Consolas, monospace;

  /* Rhythm */
  --radius-sm: 8px;
  --radius-md: 14px;
  --radius-lg: 22px;
  --gap: 24px;
  --header-h: 72px;
}

/* ---------- Reset ---------- */
* { box-sizing: border-box; }
html { scroll-behavior: smooth; scroll-padding-top: calc(var(--header-h) + 16px); }
body {
  margin: 0;
  background: var(--bg);
  color: var(--text-primary);
  font-family: var(--font-ui);
  line-height: 1.55;
  -webkit-font-smoothing: antialiased;
}
img, svg { max-width: 100%; display: block; }
a { color: inherit; text-decoration: none; }
button { font-family: inherit; cursor: pointer; }
ul { margin: 0; padding: 0; list-style: none; }
h1, h2, h3, h4, p { margin: 0; }

/* Visible focus state everywhere — accessibility floor */
:focus-visible {
  outline: 2px solid var(--accent-blue);
  outline-offset: 2px;
  border-radius: 4px;
}

@media (prefers-reduced-motion: reduce) {
  *, *::before, *::after { animation-duration: 0.001ms !important; transition-duration: 0.001ms !important; }
}

.container {
  max-width: 1180px;
  margin: 0 auto;
  padding: 0 32px;
}

.mono { font-family: var(--font-mono); }

/* Background engineering grid, used behind the hero */
.grid-field {
  position: absolute;
  inset: 0;
  background-image:
    linear-gradient(rgba(56, 189, 248, 0.06) 1px, transparent 1px),
    linear-gradient(90deg, rgba(56, 189, 248, 0.06) 1px, transparent 1px);
  background-size: 44px 44px;
  mask-image: radial-gradient(ellipse 70% 60% at 50% 30%, black 40%, transparent 100%);
  pointer-events: none;
}

/* ---------- Header ---------- */
.site-header {
  position: sticky;
  top: 0;
  z-index: 100;
  height: var(--header-h);
  display: flex;
  align-items: center;
  background: rgba(11, 17, 32, 0.82);
  backdrop-filter: blur(14px);
  border-bottom: 1px solid var(--border);
}
.site-header .container { display: flex; align-items: center; justify-content: space-between; gap: 24px; width: 100%; }
.brand { display: flex; align-items: center; gap: 10px; font-weight: 700; font-size: 1.05rem; letter-spacing: 0.01em; }
.brand-mark {
  width: 34px; height: 34px;
  display: grid; place-items: center;
  border-radius: 9px;
  background: linear-gradient(150deg, var(--accent-blue) 0%, #0369A1 100%);
  color: #04121F;
  font-family: var(--font-mono);
  font-weight: 700;
  font-size: 1.05rem;
}
.brand-name { color: var(--text-primary); }

.main-nav { display: flex; align-items: center; gap: 4px; }
.main-nav a {
  padding: 9px 14px;
  border-radius: var(--radius-sm);
  color: var(--text-secondary);
  font-size: 0.92rem;
  font-weight: 500;
  transition: color 0.15s ease, background 0.15s ease;
}
.main-nav a:hover { color: var(--text-primary); background: rgba(148, 163, 184, 0.08); }
.main-nav a.active { color: var(--accent-blue); background: rgba(56, 189, 248, 0.1); }

.header-instructor {
  font-size: 0.82rem;
  color: var(--text-secondary);
  white-space: nowrap;
}
.header-instructor strong { color: var(--text-primary); font-weight: 600; }

.menu-toggle {
  display: none;
  width: 40px; height: 40px;
  border-radius: var(--radius-sm);
  border: 1px solid var(--border);
  background: var(--card);
  align-items: center; justify-content: center;
}
.menu-toggle span, .menu-toggle span::before, .menu-toggle span::after {
  content: ''; display: block; width: 18px; height: 2px; background: var(--text-primary);
  position: relative; transition: transform 0.2s ease, opacity 0.2s ease;
}
.menu-toggle span::before { position: absolute; top: -6px; }
.menu-toggle span::after { position: absolute; top: 6px; }
.menu-toggle[aria-expanded="true"] span { background: transparent; }
.menu-toggle[aria-expanded="true"] span::before { transform: translateY(6px) rotate(45deg); }
.menu-toggle[aria-expanded="true"] span::after { transform: translateY(-6px) rotate(-45deg); }

/* ---------- Sections / page switching ---------- */
.view { display: none; }
.view.active { display: block; }
.section { padding: 88px 0; }
.section-compact { padding: 56px 0; }
.section-head { max-width: 640px; margin: 0 0 40px; }
.eyebrow { color: var(--accent-blue); font-family: var(--font-mono); font-size: 0.8rem; margin-bottom: 10px; }
.section-title { font-size: clamp(1.6rem, 2.6vw, 2.1rem); font-weight: 700; letter-spacing: -0.01em; }
.section-sub { color: var(--text-secondary); margin-top: 12px; font-size: 1.02rem; }

/* ---------- Buttons ---------- */
.btn {
  display: inline-flex; align-items: center; justify-content: center; gap: 8px;
  padding: 12px 22px;
  border-radius: var(--radius-sm);
  font-weight: 600;
  font-size: 0.94rem;
  border: 1px solid transparent;
  transition: transform 0.12s ease, background 0.15s ease, border-color 0.15s ease;
}
.btn:active { transform: translateY(1px); }
.btn-primary { background: var(--accent-blue); color: #04121F; }
.btn-primary:hover { background: #5FCBFA; }
.btn-secondary { background: transparent; border-color: var(--border-strong); color: var(--text-primary); }
.btn-secondary:hover { border-color: var(--accent-blue); color: var(--accent-blue); }
.btn-ghost { background: transparent; color: var(--text-secondary); border-color: var(--border); }
.btn-ghost:hover { color: var(--text-primary); border-color: var(--border-strong); }
.btn-sm { padding: 8px 14px; font-size: 0.85rem; }
.btn-block { width: 100%; }
.btn:disabled { opacity: 0.45; cursor: not-allowed; }

/* ---------- Hero ---------- */
.hero { position: relative; overflow: hidden; padding: 96px 0 72px; }
.hero .container { position: relative; z-index: 1; display: grid; grid-template-columns: 1.05fr 0.95fr; gap: 56px; align-items: center; }
.hero-heading { font-size: clamp(2.3rem, 4.4vw, 3.4rem); font-weight: 800; letter-spacing: -0.02em; line-height: 1.08; }
.hero-heading .accent { color: var(--accent-blue); }
.hero-text { color: var(--text-secondary); font-size: 1.08rem; margin-top: 20px; max-width: 46ch; }
.hero-actions { display: flex; gap: 14px; margin-top: 32px; flex-wrap: wrap; }
.hero-visual {
  background: var(--card);
  border: 1px solid var(--border);
  border-radius: var(--radius-lg);
  padding: 32px 24px 26px;
  box-shadow: var(--shadow);
}
.hero-visual-label { font-family: var(--font-mono); font-size: 0.78rem; color: var(--text-secondary); margin-bottom: 18px; letter-spacing: 0.03em; }
.hero-resistor-result { text-align: center; margin-top: 18px; }
.hero-resistor-result .value { font-family: var(--font-mono); font-size: 1.9rem; font-weight: 700; }
.hero-resistor-result .tol { color: var(--text-secondary); font-size: 0.95rem; margin-top: 4px; }
.hero-bands-row { display: flex; justify-content: center; gap: 10px; margin-top: 20px; font-family: var(--font-mono); font-size: 0.8rem; color: var(--text-secondary); flex-wrap: wrap; }
.hero-bands-row span { padding: 4px 10px; border-radius: 999px; background: var(--surface-2); border: 1px solid var(--border); }

/* ---------- Cards ---------- */
.card {
  background: var(--card);
  border: 1px solid var(--border);
  border-radius: var(--radius-lg);
  padding: 28px;
}
.card-title { font-size: 1.02rem; font-weight: 700; margin-bottom: 4px; }
.card-sub { color: var(--text-secondary); font-size: 0.88rem; }

/* ---------- Calculator layout ---------- */
.calc-grid { display: grid; grid-template-columns: 1.1fr 0.9fr; gap: var(--gap); align-items: start; }

.mode-switch { display: inline-flex; padding: 4px; background: var(--surface-2); border-radius: 999px; border: 1px solid var(--border); gap: 2px; }
.mode-switch button {
  padding: 9px 20px; border-radius: 999px; border: none; background: transparent;
  color: var(--text-secondary); font-weight: 600; font-size: 0.88rem; transition: all 0.15s ease;
}
.mode-switch button.active { background: var(--accent-blue); color: #04121F; }

.resistor-stage { margin: 28px 0 8px; padding: 20px 8px; background: var(--surface-2); border-radius: var(--radius-md); border: 1px solid var(--border); }
.resistor-svg { width: 100%; height: auto; }
.resistor-band { transition: fill 0.15s ease; }
.resistor-band.band-active { filter: drop-shadow(0 0 6px rgba(56,189,248,0.9)); }

.band-groups { display: flex; flex-direction: column; gap: 22px; margin-top: 24px; }
.band-group-label { display: flex; justify-content: space-between; align-items: baseline; margin-bottom: 10px; }
.band-group-label .name { font-weight: 600; font-size: 0.92rem; }
.band-group-label .selected-color { font-family: var(--font-mono); font-size: 0.82rem; color: var(--accent-blue); }
.color-grid { display: grid; grid-template-columns: repeat(6, 1fr); gap: 8px; }
.color-swatch {
  position: relative;
  display: flex; flex-direction: column; align-items: center; gap: 6px;
  padding: 10px 4px 8px;
  border-radius: var(--radius-sm);
  background: rgba(255,255,255,0.02);
  border: 1.5px solid var(--border);
  min-height: 64px;
}
.color-swatch:hover { border-color: var(--border-strong); background: rgba(255,255,255,0.05); }
.color-swatch.selected { border-color: var(--accent-blue); background: rgba(56,189,248,0.08); }
.color-swatch .dot { width: 20px; height: 20px; border-radius: 50%; border: 1px solid rgba(255,255,255,0.25); }
.color-swatch .label { font-size: 0.66rem; color: var(--text-secondary); font-weight: 600; letter-spacing: 0.01em; }
.color-swatch.selected .label { color: var(--text-primary); }
.color-swatch .val { font-family: var(--font-mono); font-size: 0.62rem; color: var(--text-secondary); position: absolute; top: 5px; right: 7px; }
.color-swatch:disabled { opacity: 0.3; cursor: not-allowed; }

.calc-actions { display: flex; gap: 10px; margin-top: 24px; flex-wrap: wrap; }

/* ---------- Result panel ---------- */
.result-panel { position: sticky; top: calc(var(--header-h) + 24px); display: flex; flex-direction: column; gap: 16px; }
.result-card { text-align: center; padding: 32px 24px; }
.result-label { font-family: var(--font-mono); font-size: 0.78rem; color: var(--text-secondary); letter-spacing: 0.05em; margin-bottom: 6px; }
.result-value { font-family: var(--font-mono); font-size: 2.4rem; font-weight: 700; color: var(--accent-green); line-height: 1.1; }
.result-row { display: flex; justify-content: space-around; margin-top: 20px; padding-top: 20px; border-top: 1px solid var(--border); }
.result-stat .stat-label { font-size: 0.76rem; color: var(--text-secondary); }
.result-stat .stat-value { font-family: var(--font-mono); font-weight: 700; margin-top: 4px; }
.result-error { color: var(--danger); font-size: 0.88rem; text-align: center; padding: 8px 0; }
.result-actions { display: flex; gap: 8px; }

.explain-card .step-list { display: flex; flex-direction: column; gap: 12px; margin-top: 16px; }
.step-item { display: flex; gap: 14px; align-items: flex-start; padding: 12px; border-radius: var(--radius-sm); background: rgba(255,255,255,0.02); border: 1px solid var(--border); }
.step-item .step-num { font-family: var(--font-mono); font-size: 0.78rem; color: var(--accent-blue); min-width: 22px; }
.step-item .step-swatch { width: 16px; height: 16px; border-radius: 50%; margin-top: 3px; flex-shrink: 0; border: 1px solid rgba(255,255,255,0.25); }
.step-item .step-body .step-label { font-size: 0.72rem; color: var(--text-secondary); text-transform: uppercase; letter-spacing: 0.04em; }
.step-item .step-body .step-text { font-family: var(--font-mono); font-size: 0.9rem; margin-top: 2px; }
.calc-line { margin-top: 18px; padding: 16px; border-radius: var(--radius-sm); background: var(--surface-2); text-align: center; font-family: var(--font-mono); font-size: 1.05rem; }
.calc-line .arrow { color: var(--text-secondary); margin: 0 8px; }
.final-answer { margin-top: 14px; text-align: center; font-family: var(--font-mono); font-size: 1.3rem; font-weight: 700; color: var(--accent-green); }

/* ---------- Reverse calculator ---------- */
.reverse-form { display: grid; gap: 16px; margin-top: 20px; }
.field-row { display: grid; grid-template-columns: 1fr 110px; gap: 10px; }
.field label { display: block; font-size: 0.82rem; color: var(--text-secondary); margin-bottom: 6px; }
.field input, .field select {
  width: 100%; padding: 12px 14px; border-radius: var(--radius-sm);
  background: var(--surface-2); border: 1px solid var(--border); color: var(--text-primary);
  font-family: var(--font-mono); font-size: 0.98rem;
}
.field input:focus, .field select:focus { border-color: var(--accent-blue); }
.segmented { display: flex; gap: 8px; }
.segmented button { flex: 1; padding: 10px; border-radius: var(--radius-sm); border: 1px solid var(--border); background: var(--surface-2); color: var(--text-secondary); font-weight: 600; font-size: 0.85rem; }
.segmented button.active { border-color: var(--accent-blue); color: var(--accent-blue); background: rgba(56,189,248,0.08); }

/* ---------- Reference table ---------- */
.ref-filters { display: flex; gap: 8px; margin-bottom: 20px; flex-wrap: wrap; }
.ref-filters button {
  padding: 8px 16px; border-radius: 999px; border: 1px solid var(--border);
  background: var(--card); color: var(--text-secondary); font-size: 0.84rem; font-weight: 600;
}
.ref-filters button.active { border-color: var(--accent-blue); color: var(--accent-blue); background: rgba(56,189,248,0.08); }
.ref-table { width: 100%; border-collapse: collapse; }
.ref-table th { text-align: left; font-size: 0.76rem; color: var(--text-secondary); text-transform: uppercase; letter-spacing: 0.04em; padding: 10px 14px; border-bottom: 1px solid var(--border-strong); }
.ref-table td { padding: 12px 14px; border-bottom: 1px solid var(--border); font-family: var(--font-mono); font-size: 0.9rem; }
.ref-color-cell { display: flex; align-items: center; gap: 10px; font-family: var(--font-ui); font-weight: 600; }
.ref-color-cell .dot { width: 16px; height: 16px; border-radius: 50%; border: 1px solid rgba(255,255,255,0.25); flex-shrink: 0; }
.ref-table tr:hover { background: rgba(255,255,255,0.02); }
.ref-table-wrap { overflow-x: auto; border: 1px solid var(--border); border-radius: var(--radius-md); }

/* ---------- Learn section ---------- */
.lesson-list { display: flex; flex-direction: column; gap: 14px; margin-top: 32px; }
.lesson {
  background: var(--card); border: 1px solid var(--border); border-radius: var(--radius-md);
  padding: 22px 24px;
}
.lesson-head { display: flex; justify-content: space-between; align-items: center; cursor: pointer; gap: 16px; }
.lesson-head h3 { font-size: 1rem; font-weight: 700; }
.lesson-toggle { width: 26px; height: 26px; border-radius: 50%; border: 1px solid var(--border-strong); display: grid; place-items: center; color: var(--text-secondary); flex-shrink: 0; transition: transform 0.2s ease; }
.lesson.open .lesson-toggle { transform: rotate(45deg); }
.lesson-body { color: var(--text-secondary); font-size: 0.95rem; margin-top: 0; max-height: 0; overflow: hidden; transition: max-height 0.25s ease, margin-top 0.25s ease; }
.lesson.open .lesson-body { margin-top: 16px; max-height: 900px; }
.lesson-body p { margin-bottom: 10px; }
.lesson-body p:last-child { margin-bottom: 0; }
.lesson-example { margin-top: 10px; padding: 12px 14px; background: var(--surface-2); border-radius: var(--radius-sm); font-family: var(--font-mono); font-size: 0.88rem; color: var(--text-primary); }

/* ---------- Practice ---------- */
.practice-setup { text-align: center; padding: 56px 32px; }
.difficulty-options { display: flex; justify-content: center; gap: 14px; margin-top: 24px; flex-wrap: wrap; }
.difficulty-card {
  width: 168px; padding: 22px 16px; border-radius: var(--radius-md);
  background: var(--surface-2); border: 1.5px solid var(--border);
  text-align: center;
}
.difficulty-card:hover, .difficulty-card:focus-visible { border-color: var(--accent-blue); }
.difficulty-card .diff-name { font-weight: 700; }
.difficulty-card .diff-detail { font-size: 0.8rem; color: var(--text-secondary); margin-top: 6px; }

.quiz-head { display: flex; justify-content: space-between; align-items: center; margin-bottom: 18px; font-family: var(--font-mono); font-size: 0.85rem; color: var(--text-secondary); }
.progress-bar { height: 6px; border-radius: 999px; background: var(--surface-2); overflow: hidden; margin-bottom: 28px; }
.progress-fill { height: 100%; background: var(--accent-blue); transition: width 0.3s ease; }
.quiz-question-title { text-align: center; font-size: 1.1rem; font-weight: 600; margin-bottom: 24px; }
.quiz-resistor { max-width: 420px; margin: 0 auto 28px; }
.quiz-options { display: grid; grid-template-columns: 1fr 1fr; gap: 12px; max-width: 640px; margin: 0 auto; }
.quiz-option {
  padding: 16px; border-radius: var(--radius-sm); border: 1.5px solid var(--border);
  background: var(--surface-2); font-family: var(--font-mono); font-size: 0.95rem; text-align: left;
}
.quiz-option:hover:not(:disabled) { border-color: var(--border-strong); }
.quiz-option.correct { border-color: var(--accent-green); background: rgba(34,197,94,0.12); color: var(--accent-green); }
.quiz-option.incorrect { border-color: var(--danger); background: rgba(239,68,68,0.12); color: var(--danger); }
.quiz-option:disabled { cursor: default; }
.quiz-feedback { text-align: center; margin-top: 24px; }
.quiz-feedback .verdict { font-size: 1.15rem; font-weight: 700; }
.quiz-feedback .verdict.good { color: var(--accent-green); }
.quiz-feedback .verdict.bad { color: var(--danger); }
.quiz-feedback .explain { color: var(--text-secondary); margin-top: 8px; font-size: 0.9rem; }
.quiz-feedback .btn { margin-top: 20px; }

.quiz-complete { text-align: center; padding: 40px 20px; }
.quiz-score { font-family: var(--font-mono); font-size: 3rem; font-weight: 700; color: var(--accent-blue); margin: 12px 0; }
.quiz-stats-row { display: flex; justify-content: center; gap: 32px; margin: 20px 0 28px; }
.quiz-stats-row .stat { text-align: center; }
.quiz-stats-row .stat-num { font-family: var(--font-mono); font-size: 1.4rem; font-weight: 700; }
.quiz-stats-row .stat-label { font-size: 0.78rem; color: var(--text-secondary); }

/* ---------- About ---------- */
.about-grid { display: grid; grid-template-columns: 1.3fr 0.7fr; gap: var(--gap); align-items: start; }
.about-focus-list { display: flex; flex-direction: column; gap: 10px; margin-top: 16px; }
.about-focus-list li { padding: 10px 14px; border-radius: var(--radius-sm); background: var(--surface-2); font-size: 0.9rem; border: 1px solid var(--border); }

/* ---------- Footer ---------- */
.site-footer { border-top: 1px solid var(--border); padding: 48px 0 32px; margin-top: 40px; }
.footer-top { display: flex; justify-content: space-between; align-items: flex-start; flex-wrap: wrap; gap: 24px; }
.footer-brand .brand { margin-bottom: 8px; }
.footer-tagline { color: var(--text-secondary); font-size: 0.9rem; }
.footer-nav { display: flex; gap: 20px; flex-wrap: wrap; }
.footer-nav a { color: var(--text-secondary); font-size: 0.88rem; }
.footer-nav a:hover { color: var(--accent-blue); }
.footer-meta { margin-top: 32px; padding-top: 20px; border-top: 1px solid var(--border); color: var(--text-secondary); font-size: 0.8rem; display: flex; justify-content: space-between; flex-wrap: wrap; gap: 8px; }

/* ---------- Toast ---------- */
.toast {
  position: fixed; bottom: 24px; left: 50%; transform: translateX(-50%) translateY(20px);
  background: var(--surface-2); border: 1px solid var(--border-strong); color: var(--text-primary);
  padding: 12px 20px; border-radius: var(--radius-sm); font-size: 0.88rem; font-weight: 600;
  opacity: 0; pointer-events: none; transition: opacity 0.2s ease, transform 0.2s ease; z-index: 200;
}
.toast.show { opacity: 1; transform: translateX(-50%) translateY(0); }

/* ---------- Utility ---------- */
.visually-hidden { position: absolute; width: 1px; height: 1px; overflow: hidden; clip: rect(0,0,0,0); white-space: nowrap; }
.two-col-lessons { display: grid; grid-template-columns: 1fr; gap: 14px; }