/*
 * hkp5exam.hk — Math Fractions Practice Tool
 * style.css
 *
 * NOTE: Font loading — production should self-host Noto Sans TC and Noto Serif TC
 * from Google Fonts (download via google-webfonts-helper or fontsource) to comply
 * with the strict CSP (style-src 'self'). The current font stack uses system fonts
 * as a safe fallback: PingFang TC (macOS/iOS), Microsoft JhengHei (Windows).
 *
 * Design tokens mirror the parent hkp5exam.hk site.
 */

/* ============================================================
   DESIGN TOKENS
   ============================================================ */
:root {
  /* Brand colours */
  --c-navy:        #0B2545;
  --c-orange:      #F4A261;
  --c-cream:       #FDF6EC;
  --c-bg:          #FBFAF7;
  --c-text:        #1A1F2E;
  --c-text-muted:  #5C6478;
  --c-border:      #E4E1D9;

  /* Semantic */
  --c-surface:     #FFFFFF;
  --c-surface-alt: var(--c-cream);
  --c-accent:      var(--c-orange);
  --c-navy-soft:   #1C3D6E;
  --c-correct:     #2D7D46;
  --c-correct-bg:  #EAF7EE;
  --c-wrong:       #C0392B;
  --c-wrong-bg:    #FDECEA;

  /* Typography */
  --font-serif: 'Noto Serif TC', 'Georgia', 'Times New Roman', serif;
  --font-sans:  'Noto Sans TC', 'PingFang TC', 'Microsoft JhengHei', 'Heiti TC', 'Apple LiGothic Medium', sans-serif;

  /* Spacing */
  --space-1:  4px;
  --space-2:  8px;
  --space-3:  12px;
  --space-4:  16px;
  --space-5:  20px;
  --space-6:  24px;
  --space-8:  32px;
  --space-10: 40px;
  --space-12: 48px;
  --space-16: 64px;
  --space-20: 80px;
  --space-24: 96px;

  /* Shape */
  --radius:    12px;
  --radius-lg: 20px;
  --radius-sm: 6px;

  /* Elevation */
  --shadow-sm: 0 2px 8px rgba(11,37,69,0.07);
  --shadow-md: 0 4px 16px rgba(11,37,69,0.10);
  --shadow-lg: 0 8px 32px rgba(11,37,69,0.14);

  /* Motion */
  --transition-fast:   150ms ease;
  --transition-base:   250ms ease;
  --transition-slow:   400ms ease;

  /* Layout */
  --max-content: 1160px;
  --nav-height:  64px;
}

/* Dark mode token overrides */
[data-theme="dark"] {
  --c-bg:          #0E1520;
  --c-surface:     #162030;
  --c-surface-alt: #1C2D42;
  --c-cream:       #1C2D42;
  --c-text:        #E8EDF5;
  --c-text-muted:  #8A97AE;
  --c-border:      #2A3A52;
  --c-navy:        #4A7EBF;
  --c-navy-soft:   #3A6BA8;
  --c-orange:      #F4A261;
  --c-correct:     #4CAF70;
  --c-correct-bg:  #0E2918;
  --c-wrong:       #E05A4E;
  --c-wrong-bg:    #2D1210;
}

/* ============================================================
   RESET & BASE
   ============================================================ */
*, *::before, *::after {
  box-sizing: border-box;
  margin: 0;
  padding: 0;
}

html {
  scroll-behavior: smooth;
  font-size: 16px;
  -webkit-text-size-adjust: 100%;
}

body {
  font-family: var(--font-sans);
  font-size: 1rem;
  line-height: 1.7;
  color: var(--c-text);
  background: var(--c-bg);
  transition: background var(--transition-base), color var(--transition-base);
}

/* Scroll lock when auth is locked */
body[data-auth="locked"] {
  overflow: hidden;
}

/* Hide site content when locked */
body[data-auth="locked"] .site-header,
body[data-auth="locked"] main,
body[data-auth="locked"] .site-footer,
body[data-auth="locked"] .skip-link {
  visibility: hidden;
  pointer-events: none;
}

/* Show auth screen only when locked */
body[data-auth="unlocked"] #auth-screen {
  display: none;
}

img, svg {
  display: block;
  max-width: 100%;
}

button {
  font-family: inherit;
  cursor: pointer;
  border: none;
  background: none;
}

input, select, textarea {
  font-family: inherit;
  font-size: inherit;
  color: inherit;
}

ul, ol {
  padding-left: var(--space-6);
}

a {
  color: inherit;
  text-decoration: none;
}

/* ============================================================
   TYPOGRAPHY HELPERS
   ============================================================ */
.eyebrow {
  font-family: var(--font-sans);
  font-size: 0.8125rem;
  font-weight: 600;
  letter-spacing: 0.08em;
  text-transform: uppercase;
  color: var(--c-orange);
  margin-bottom: var(--space-2);
}

h1, h2, h3 {
  font-family: var(--font-serif);
  line-height: 1.35;
  color: var(--c-text);
}

h1 { font-size: clamp(1.6rem, 4vw, 2.25rem); }
h2 { font-size: clamp(1.3rem, 3vw, 1.75rem); }
h3 { font-size: clamp(1.05rem, 2vw, 1.2rem); }

p { margin-top: var(--space-3); }
p:first-child { margin-top: 0; }

strong { font-weight: 700; }

/* ============================================================
   LAYOUT UTILITY
   ============================================================ */
.container {
  width: 100%;
  max-width: var(--max-content);
  margin-inline: auto;
  padding-inline: var(--space-6);
}

.section-heading {
  text-align: center;
  margin-bottom: var(--space-12);
}

.section-heading p { max-width: 60ch; margin-inline: auto; }
.section-heading.compact { margin-bottom: var(--space-8); }

/* ============================================================
   SKIP LINK
   ============================================================ */
.skip-link {
  position: fixed;
  top: var(--space-4);
  left: var(--space-4);
  z-index: 9999;
  padding: var(--space-2) var(--space-4);
  background: var(--c-navy);
  color: #FFF;
  border-radius: var(--radius-sm);
  font-size: 0.875rem;
  transform: translateY(-200%);
  transition: transform var(--transition-fast);
}

.skip-link:focus {
  transform: translateY(0);
  outline: 2px solid var(--c-orange);
  outline-offset: 2px;
}

/* ============================================================
   AUTH SCREEN
   ============================================================ */
.auth-screen {
  position: fixed;
  inset: 0;
  z-index: 9000;
  background: var(--c-bg);
  display: flex;
  align-items: center;
  justify-content: center;
  padding: var(--space-6);
  transition: background var(--transition-base);
}

.auth-card {
  width: 100%;
  max-width: 420px;
  background: var(--c-surface);
  border: 1px solid var(--c-border);
  border-radius: var(--radius-lg);
  padding: var(--space-10) var(--space-8);
  box-shadow: var(--shadow-lg);
}

.auth-brand {
  display: flex;
  align-items: center;
  gap: var(--space-3);
  margin-bottom: var(--space-6);
  font-family: var(--font-serif);
  font-size: 1.1rem;
  font-weight: 700;
  color: var(--c-navy);
}

[data-theme="dark"] .auth-brand {
  color: var(--c-orange);
}

.auth-brand .brand-mark {
  width: 44px;
  height: 44px;
  color: var(--c-orange);
  flex-shrink: 0;
}

.auth-card .eyebrow { margin-bottom: var(--space-1); }
.auth-card h1 {
  font-size: clamp(1.2rem, 3vw, 1.5rem);
  margin-bottom: var(--space-2);
}

.auth-note {
  font-size: 0.875rem;
  color: var(--c-text-muted);
  margin-bottom: var(--space-6);
}

/* ============================================================
   LOGIN FORM
   ============================================================ */
.login-form {
  display: flex;
  flex-direction: column;
  gap: var(--space-3);
}

.login-form label {
  font-size: 0.875rem;
  font-weight: 600;
  color: var(--c-text-muted);
  margin-bottom: calc(var(--space-1) * -1);
}

.login-form input {
  width: 100%;
  padding: var(--space-3) var(--space-4);
  border: 1.5px solid var(--c-border);
  border-radius: var(--radius-sm);
  background: var(--c-bg);
  color: var(--c-text);
  font-size: 1rem;
  transition: border-color var(--transition-fast), box-shadow var(--transition-fast);
}

.login-form input:focus {
  outline: none;
  border-color: var(--c-navy);
  box-shadow: 0 0 0 3px rgba(11,37,69,0.12);
}

[data-theme="dark"] .login-form input:focus {
  box-shadow: 0 0 0 3px rgba(74,126,191,0.25);
}

.login-form .primary-button {
  margin-top: var(--space-2);
}

.login-message {
  font-size: 0.875rem;
  font-weight: 600;
  color: var(--c-wrong);
  min-height: 1.4em;
  text-align: center;
}

/* ============================================================
   BUTTONS
   ============================================================ */
.primary-button {
  display: inline-flex;
  align-items: center;
  justify-content: center;
  gap: var(--space-2);
  padding: var(--space-3) var(--space-6);
  background: var(--c-navy);
  color: #FFF;
  border-radius: var(--radius-sm);
  font-size: 0.9375rem;
  font-weight: 600;
  letter-spacing: 0.01em;
  transition: background var(--transition-fast), transform var(--transition-fast), box-shadow var(--transition-fast);
  cursor: pointer;
  border: none;
}

.primary-button:hover {
  background: var(--c-navy-soft);
  transform: translateY(-1px);
  box-shadow: var(--shadow-md);
}

.primary-button:active {
  transform: translateY(0);
  box-shadow: none;
}

.primary-button:focus-visible {
  outline: 2px solid var(--c-orange);
  outline-offset: 3px;
}

.secondary-button {
  display: inline-flex;
  align-items: center;
  justify-content: center;
  gap: var(--space-2);
  padding: var(--space-3) var(--space-6);
  background: transparent;
  color: var(--c-navy);
  border: 2px solid var(--c-navy);
  border-radius: var(--radius-sm);
  font-size: 0.9375rem;
  font-weight: 600;
  letter-spacing: 0.01em;
  transition: background var(--transition-fast), color var(--transition-fast), transform var(--transition-fast);
  cursor: pointer;
}

.secondary-button:hover {
  background: var(--c-navy);
  color: #FFF;
  transform: translateY(-1px);
}

.secondary-button:active { transform: translateY(0); }

.secondary-button:focus-visible {
  outline: 2px solid var(--c-orange);
  outline-offset: 3px;
}

[data-theme="dark"] .secondary-button {
  color: var(--c-orange);
  border-color: var(--c-orange);
}

[data-theme="dark"] .secondary-button:hover {
  background: var(--c-orange);
  color: #0E1520;
}

/* ============================================================
   SITE HEADER & NAV
   ============================================================ */
.site-header {
  position: sticky;
  top: 0;
  z-index: 100;
  height: var(--nav-height);
  background: rgba(251,250,247,0.92);
  backdrop-filter: blur(12px);
  -webkit-backdrop-filter: blur(12px);
  border-bottom: 1px solid var(--c-border);
  transition: background var(--transition-base), border-color var(--transition-base);
}

[data-theme="dark"] .site-header {
  background: rgba(14,21,32,0.92);
}

.nav {
  height: 100%;
  max-width: var(--max-content);
  margin-inline: auto;
  padding-inline: var(--space-6);
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: var(--space-4);
}

.brand {
  display: flex;
  align-items: center;
  gap: var(--space-3);
  font-family: var(--font-serif);
  font-size: 1rem;
  font-weight: 700;
  color: var(--c-navy);
  text-decoration: none;
  white-space: nowrap;
}

[data-theme="dark"] .brand { color: var(--c-orange); }

.brand-mark {
  width: 26px;
  height: 26px;
  color: var(--c-orange);
  flex-shrink: 0;
}

.nav-actions {
  display: flex;
  align-items: center;
  gap: var(--space-1);
}

.nav-link {
  padding: var(--space-2) var(--space-3);
  font-size: 0.875rem;
  font-weight: 600;
  color: var(--c-text-muted);
  border-radius: var(--radius-sm);
  transition: color var(--transition-fast), background var(--transition-fast);
  white-space: nowrap;
}

.nav-link:hover {
  color: var(--c-text);
  background: rgba(11,37,69,0.06);
}

[data-theme="dark"] .nav-link:hover {
  background: rgba(255,255,255,0.08);
}

.nav-link:focus-visible {
  outline: 2px solid var(--c-orange);
  outline-offset: 2px;
}

.theme-toggle {
  padding: var(--space-2) var(--space-4);
  font-size: 0.8125rem;
  font-weight: 600;
  color: var(--c-navy);
  background: var(--c-cream);
  border: 1.5px solid var(--c-border);
  border-radius: var(--radius-sm);
  transition: background var(--transition-fast), color var(--transition-fast), border-color var(--transition-fast);
  white-space: nowrap;
}

.theme-toggle:hover {
  background: var(--c-border);
}

[data-theme="dark"] .theme-toggle {
  color: var(--c-orange);
  background: var(--c-surface-alt);
  border-color: var(--c-border);
}

.theme-toggle:focus-visible {
  outline: 2px solid var(--c-orange);
  outline-offset: 2px;
}

/* ============================================================
   HERO
   ============================================================ */
.hero {
  max-width: var(--max-content);
  margin-inline: auto;
  padding: var(--space-20) var(--space-6) var(--space-16);
  display: grid;
  grid-template-columns: 1fr auto;
  align-items: center;
  gap: var(--space-12);
}

.hero-copy { max-width: 56ch; }

.hero-copy h1 {
  margin-bottom: var(--space-4);
  margin-top: var(--space-3);
}

.hero-text {
  font-size: 1.0625rem;
  color: var(--c-text-muted);
  max-width: 52ch;
}

.hero-actions {
  display: flex;
  flex-wrap: wrap;
  gap: var(--space-3);
  margin-top: var(--space-8);
}

.hero-card {
  background: var(--c-surface);
  border: 1px solid var(--c-border);
  border-radius: var(--radius-lg);
  padding: var(--space-8);
  box-shadow: var(--shadow-md);
  min-width: 220px;
  text-align: center;
}

.hero-card p {
  font-size: 0.875rem;
  color: var(--c-text-muted);
  margin-top: var(--space-4);
}

.fraction-demo {
  display: flex;
  align-items: center;
  justify-content: center;
  gap: var(--space-3);
  font-family: var(--font-serif);
  font-size: 1.5rem;
  font-weight: 700;
  color: var(--c-navy);
  margin-bottom: var(--space-5);
}

[data-theme="dark"] .fraction-demo { color: var(--c-orange); }

.quarter-grid {
  display: grid;
  grid-template-columns: repeat(2, 1fr);
  gap: var(--space-2);
  width: 80px;
  margin-inline: auto;
}

.quarter-grid span {
  aspect-ratio: 1;
  border-radius: var(--space-1);
  background: var(--c-border);
  border: 2px solid transparent;
  transition: background var(--transition-base);
}

.quarter-grid span.filled {
  background: var(--c-orange);
}

/* ============================================================
   SOURCE FOCUS
   ============================================================ */
.source-focus {
  background: var(--c-cream);
  padding: var(--space-16) var(--space-6);
  transition: background var(--transition-base);
}

.source-focus > div:first-child {
  max-width: var(--max-content);
  margin-inline: auto;
  text-align: center;
  margin-bottom: var(--space-8);
}

.focus-list {
  max-width: var(--max-content);
  margin-inline: auto;
  display: grid;
  grid-template-columns: repeat(4, 1fr);
  gap: var(--space-5);
}

.focus-list article {
  background: var(--c-surface);
  border: 1px solid var(--c-border);
  border-radius: var(--radius);
  padding: var(--space-6);
  transition: box-shadow var(--transition-fast), transform var(--transition-fast);
}

.focus-list article:hover {
  box-shadow: var(--shadow-md);
  transform: translateY(-2px);
}

.focus-list h3 {
  font-size: 1rem;
  color: var(--c-navy);
  margin-bottom: var(--space-2);
}

[data-theme="dark"] .focus-list h3 { color: var(--c-orange); }

.focus-list p {
  font-size: 0.875rem;
  color: var(--c-text-muted);
}

/* ============================================================
   LESSON SECTION
   ============================================================ */
.lesson {
  max-width: var(--max-content);
  margin-inline: auto;
  padding: var(--space-20) var(--space-6);
}

.concept-layout {
  display: grid;
  grid-template-columns: 2fr 1fr 1fr;
  gap: var(--space-6);
  align-items: start;
}

.concept-card {
  background: var(--c-surface);
  border: 1px solid var(--c-border);
  border-radius: var(--radius-lg);
  padding: var(--space-8);
  transition: box-shadow var(--transition-fast);
}

.concept-card:hover { box-shadow: var(--shadow-md); }

.concept-card.large {
  border-left: 4px solid var(--c-orange);
}

.card-label {
  display: inline-block;
  font-size: 0.75rem;
  font-weight: 700;
  letter-spacing: 0.08em;
  text-transform: uppercase;
  color: var(--c-orange);
  background: rgba(244,162,97,0.12);
  padding: 2px var(--space-3);
  border-radius: 999px;
  margin-bottom: var(--space-3);
}

.concept-card h3 { margin-bottom: var(--space-3); }

/* BAR MODEL */
.bar-model {
  display: grid;
  grid-template-columns: repeat(4, 1fr);
  gap: var(--space-2);
  margin-top: var(--space-5);
  height: 40px;
}

.bar-model span {
  border-radius: var(--radius-sm);
  background: var(--c-border);
  transition: background var(--transition-base);
}

.bar-model span.active {
  background: var(--c-orange);
}

/* NUMBER LINE */
.number-line {
  display: flex;
  justify-content: space-between;
  align-items: flex-end;
  margin-top: var(--space-5);
  padding-bottom: var(--space-3);
  border-bottom: 3px solid var(--c-navy);
  position: relative;
  font-size: 0.8125rem;
  font-weight: 600;
  color: var(--c-text-muted);
}

[data-theme="dark"] .number-line { border-bottom-color: var(--c-navy); }

.number-line .point {
  position: relative;
  color: var(--c-navy);
}

[data-theme="dark"] .number-line .point { color: var(--c-orange); }

.number-line .point::after {
  content: '';
  position: absolute;
  bottom: calc(-1 * var(--space-3) - 3px);
  left: 50%;
  transform: translateX(-50%);
  width: 10px;
  height: 10px;
  border-radius: 50%;
  background: var(--c-navy);
}

[data-theme="dark"] .number-line .point::after { background: var(--c-orange); }

.number-line .quarter { left: 25%; }
.number-line .half    { left: 50%; }

/* PERCENT PILL */
.percent-pill {
  margin-top: var(--space-5);
  display: inline-block;
  padding: var(--space-3) var(--space-6);
  background: rgba(11,37,69,0.07);
  border: 1.5px solid var(--c-border);
  border-radius: 999px;
  font-family: var(--font-serif);
  font-size: 1.25rem;
  font-weight: 700;
  color: var(--c-navy);
  text-align: center;
  width: 100%;
}

[data-theme="dark"] .percent-pill {
  background: rgba(74,126,191,0.12);
  color: var(--c-orange);
}

/* ============================================================
   CONVERSION SECTION
   ============================================================ */
.conversion {
  background: var(--c-navy);
  padding: var(--space-16) var(--space-6);
  transition: background var(--transition-base);
}

[data-theme="dark"] .conversion { background: var(--c-surface); }

.conversion .section-heading { margin-bottom: var(--space-10); }

.conversion .eyebrow { color: var(--c-orange); }

.conversion .section-heading h2 {
  color: #FFF;
}

[data-theme="dark"] .conversion .section-heading h2 { color: var(--c-text); }

.conversion-grid {
  max-width: var(--max-content);
  margin-inline: auto;
  display: grid;
  grid-template-columns: repeat(4, 1fr);
  gap: var(--space-5);
}

.conversion-grid article {
  background: rgba(255,255,255,0.07);
  border: 1px solid rgba(255,255,255,0.12);
  border-radius: var(--radius);
  padding: var(--space-6);
}

[data-theme="dark"] .conversion-grid article {
  background: var(--c-surface-alt);
  border-color: var(--c-border);
}

.conversion-grid h3 {
  color: var(--c-orange);
  font-size: 1rem;
  margin-bottom: var(--space-3);
}

.conversion-grid ul {
  list-style: none;
  padding: 0;
}

.conversion-grid li {
  color: rgba(255,255,255,0.85);
  font-size: 0.875rem;
  padding: var(--space-2) 0;
  border-bottom: 1px solid rgba(255,255,255,0.1);
}

.conversion-grid li:last-child { border-bottom: none; }

[data-theme="dark"] .conversion-grid li {
  color: var(--c-text-muted);
  border-bottom-color: var(--c-border);
}

/* ============================================================
   WORKED EXAMPLE
   ============================================================ */
.worked-example {
  max-width: var(--max-content);
  margin-inline: auto;
  padding: var(--space-16) var(--space-6);
  display: grid;
  grid-template-columns: 1fr 2fr;
  gap: var(--space-12);
  align-items: start;
}

.worked-example > div { /* heading wrapper */}

.steps {
  padding-left: 0;
  list-style: none;
  counter-reset: step-counter;
}

.steps li {
  counter-increment: step-counter;
  position: relative;
  padding: var(--space-5) var(--space-5) var(--space-5) var(--space-12);
  background: var(--c-surface);
  border: 1px solid var(--c-border);
  border-radius: var(--radius);
  margin-bottom: var(--space-3);
  font-size: 0.9375rem;
}

.steps li::before {
  content: counter(step-counter);
  position: absolute;
  left: var(--space-4);
  top: 50%;
  transform: translateY(-50%);
  width: 28px;
  height: 28px;
  border-radius: 50%;
  background: var(--c-navy);
  color: #FFF;
  font-size: 0.8125rem;
  font-weight: 700;
  display: flex;
  align-items: center;
  justify-content: center;
}

/* ============================================================
   TOOLS SECTION
   ============================================================ */
.tools-section {
  background: var(--c-cream);
  padding: var(--space-20) var(--space-6);
  transition: background var(--transition-base);
}

.tools-section .section-heading {
  max-width: var(--max-content);
  margin-inline: auto;
  margin-bottom: var(--space-10);
  text-align: left;
}

.tool-grid {
  max-width: var(--max-content);
  margin-inline: auto;
  display: grid;
  grid-template-columns: repeat(3, 1fr);
  gap: var(--space-6);
}

.tool-card {
  background: var(--c-surface);
  border: 1px solid var(--c-border);
  border-radius: var(--radius-lg);
  padding: var(--space-8);
  box-shadow: var(--shadow-sm);
  transition: box-shadow var(--transition-fast), transform var(--transition-fast);
}

.tool-card:hover {
  box-shadow: var(--shadow-md);
  transform: translateY(-2px);
}

.tool-card h3 {
  font-size: 1.1rem;
  color: var(--c-navy);
  margin-bottom: var(--space-2);
}

[data-theme="dark"] .tool-card h3 { color: var(--c-orange); }

.tool-card > p {
  font-size: 0.875rem;
  color: var(--c-text-muted);
  margin-bottom: var(--space-5);
}

/* TIP STACK */
.tip-stack {
  display: flex;
  flex-direction: column;
  gap: var(--space-2);
}

.tip-button {
  width: 100%;
  padding: var(--space-3) var(--space-4);
  text-align: left;
  font-size: 0.875rem;
  font-weight: 600;
  color: var(--c-navy);
  background: var(--c-bg);
  border: 1.5px solid var(--c-border);
  border-radius: var(--radius-sm);
  display: flex;
  align-items: center;
  justify-content: space-between;
  transition: background var(--transition-fast), border-color var(--transition-fast);
  cursor: pointer;
}

.tip-button::after {
  content: '+';
  font-size: 1.1rem;
  font-weight: 400;
  color: var(--c-orange);
  flex-shrink: 0;
  transition: transform var(--transition-fast);
}

.tip-button[aria-expanded="true"]::after {
  content: '-';
}

.tip-button:hover {
  background: var(--c-cream);
  border-color: var(--c-orange);
}

.tip-button:focus-visible {
  outline: 2px solid var(--c-orange);
  outline-offset: 2px;
}

[data-theme="dark"] .tip-button {
  color: var(--c-text);
  background: var(--c-surface-alt);
}

.tip-panel {
  display: none;
  padding: var(--space-3) var(--space-4);
  background: var(--c-navy);
  color: rgba(255,255,255,0.9);
  border-radius: var(--radius-sm);
  font-size: 0.875rem;
  line-height: 1.6;
  margin-top: calc(var(--space-1) * -1);
}

.tip-panel.open {
  display: block;
}

[data-theme="dark"] .tip-panel {
  background: var(--c-surface-alt);
  border: 1px solid var(--c-border);
  color: var(--c-text);
}

/* SLIDER */
.slider-label {
  display: block;
  font-size: 0.8125rem;
  font-weight: 600;
  color: var(--c-text-muted);
  margin-top: var(--space-4);
  margin-bottom: var(--space-2);
}

#percent-slider {
  width: 100%;
  accent-color: var(--c-navy);
  cursor: pointer;
  height: 6px;
}

.tool-output {
  font-family: var(--font-serif);
  font-size: 1rem;
  font-weight: 700;
  color: var(--c-navy);
  background: rgba(11,37,69,0.06);
  border: 1px solid var(--c-border);
  border-radius: var(--radius-sm);
  padding: var(--space-3) var(--space-4);
  margin-top: var(--space-3);
  word-break: break-word;
}

[data-theme="dark"] .tool-output {
  color: var(--c-orange);
  background: rgba(74,126,191,0.1);
}

/* INTERACTIVE GRID */
.interactive-grid {
  display: grid;
  grid-template-columns: repeat(10, 1fr);
  gap: 2px;
  margin-top: var(--space-4);
  border: 2px solid var(--c-border);
  border-radius: var(--radius-sm);
  overflow: hidden;
  padding: 3px;
  background: var(--c-bg);
}

.grid-cell {
  aspect-ratio: 1;
  border-radius: 2px;
  background: var(--c-border);
  transition: background var(--transition-fast);
}

.grid-cell.filled {
  background: var(--c-orange);
}

/* CONVERTER */
.converter-row {
  display: flex;
  gap: var(--space-2);
  flex-wrap: wrap;
  margin-top: var(--space-4);
}

.converter-row select {
  padding: var(--space-3) var(--space-3);
  border: 1.5px solid var(--c-border);
  border-radius: var(--radius-sm);
  background: var(--c-bg);
  font-size: 0.875rem;
  flex-shrink: 0;
  cursor: pointer;
  transition: border-color var(--transition-fast);
}

.converter-row select:focus {
  outline: none;
  border-color: var(--c-navy);
}

.converter-row input {
  flex: 1;
  min-width: 80px;
  padding: var(--space-3) var(--space-3);
  border: 1.5px solid var(--c-border);
  border-radius: var(--radius-sm);
  background: var(--c-bg);
  font-size: 0.875rem;
  transition: border-color var(--transition-fast);
}

.converter-row input:focus {
  outline: none;
  border-color: var(--c-navy);
}

.converter-row .primary-button {
  padding: var(--space-3) var(--space-5);
  font-size: 0.875rem;
}

/* ============================================================
   QUIZ SECTION
   ============================================================ */
.quiz-section {
  max-width: var(--max-content);
  margin-inline: auto;
  padding: var(--space-20) var(--space-6) var(--space-24);
}

.quiz-intro {
  display: flex;
  justify-content: space-between;
  align-items: flex-start;
  gap: var(--space-8);
  margin-bottom: var(--space-8);
  flex-wrap: wrap;
}

.quiz-intro > div:first-child { max-width: 60ch; }

.quiz-intro h2 { margin-bottom: var(--space-2); }
.quiz-intro p  { font-size: 0.9375rem; color: var(--c-text-muted); }

.score-card {
  background: var(--c-navy);
  color: #FFF;
  border-radius: var(--radius-lg);
  padding: var(--space-6) var(--space-8);
  text-align: center;
  min-width: 140px;
  flex-shrink: 0;
  box-shadow: var(--shadow-md);
}

.score-card span {
  display: block;
  font-size: 0.8125rem;
  font-weight: 600;
  letter-spacing: 0.06em;
  opacity: 0.8;
  margin-bottom: var(--space-1);
}

.score-card strong {
  display: block;
  font-family: var(--font-serif);
  font-size: 2rem;
  color: var(--c-orange);
}

[data-theme="dark"] .score-card {
  background: var(--c-surface);
  border: 1px solid var(--c-border);
}

/* WEAKNESS CARD */
.weakness-card {
  background: var(--c-surface);
  border: 1.5px solid var(--c-border);
  border-radius: var(--radius);
  padding: var(--space-5) var(--space-6);
  margin-bottom: var(--space-8);
  display: flex;
  gap: var(--space-4);
  align-items: flex-start;
  flex-wrap: wrap;
}

.weakness-card strong {
  font-size: 0.875rem;
  font-weight: 700;
  color: var(--c-navy);
  white-space: nowrap;
  margin-top: 2px;
  flex-shrink: 0;
}

[data-theme="dark"] .weakness-card strong { color: var(--c-orange); }

.weakness-card p {
  font-size: 0.875rem;
  color: var(--c-text-muted);
  margin: 0;
}

/* QUIZ LIST */
.quiz-list {
  display: flex;
  flex-direction: column;
  gap: var(--space-5);
}

/* QUIZ ITEM (generated by JS) */
.quiz-item {
  background: var(--c-surface);
  border: 1.5px solid var(--c-border);
  border-radius: var(--radius-lg);
  padding: var(--space-7) var(--space-8);
  transition: border-color var(--transition-fast);
}

.quiz-item.answered-correct {
  border-color: var(--c-correct);
  background: var(--c-correct-bg);
}

.quiz-item.answered-wrong {
  border-color: var(--c-wrong);
  background: var(--c-wrong-bg);
}

.quiz-item-header {
  display: flex;
  align-items: flex-start;
  gap: var(--space-4);
  margin-bottom: var(--space-5);
}

.quiz-item-num {
  display: inline-flex;
  align-items: center;
  justify-content: center;
  width: 28px;
  height: 28px;
  border-radius: 50%;
  background: var(--c-navy);
  color: #FFF;
  font-size: 0.8125rem;
  font-weight: 700;
  flex-shrink: 0;
  margin-top: 3px;
}

.answered-correct .quiz-item-num { background: var(--c-correct); }
.answered-wrong   .quiz-item-num { background: var(--c-wrong); }

.quiz-item-topic {
  font-size: 0.75rem;
  font-weight: 600;
  letter-spacing: 0.06em;
  text-transform: uppercase;
  color: var(--c-orange);
  margin-bottom: var(--space-1);
}

.quiz-item-q {
  font-size: 1.0625rem;
  font-weight: 600;
  color: var(--c-text);
  line-height: 1.5;
}

.quiz-options {
  display: grid;
  grid-template-columns: 1fr 1fr;
  gap: var(--space-3);
  margin-bottom: var(--space-4);
}

.quiz-option {
  padding: var(--space-3) var(--space-4);
  text-align: left;
  background: var(--c-bg);
  border: 1.5px solid var(--c-border);
  border-radius: var(--radius-sm);
  font-size: 0.9375rem;
  color: var(--c-text);
  cursor: pointer;
  transition: background var(--transition-fast), border-color var(--transition-fast), color var(--transition-fast);
  line-height: 1.5;
}

.quiz-option:hover:not(:disabled) {
  background: var(--c-cream);
  border-color: var(--c-navy);
}

.quiz-option:disabled { cursor: default; }

.quiz-option.correct {
  background: var(--c-correct-bg);
  border-color: var(--c-correct);
  color: var(--c-correct);
  font-weight: 700;
}

.quiz-option.wrong {
  background: var(--c-wrong-bg);
  border-color: var(--c-wrong);
  color: var(--c-wrong);
}

.quiz-option.reveal {
  background: var(--c-correct-bg);
  border-color: var(--c-correct);
  color: var(--c-correct);
  font-weight: 700;
}

.quiz-feedback {
  display: none;
  padding: var(--space-3) var(--space-4);
  border-radius: var(--radius-sm);
  font-size: 0.875rem;
  line-height: 1.6;
  margin-top: var(--space-2);
}

.quiz-feedback.show { display: block; }

.quiz-feedback.correct-msg {
  background: var(--c-correct-bg);
  color: var(--c-correct);
  border: 1px solid var(--c-correct);
}

.quiz-feedback.wrong-msg {
  background: var(--c-wrong-bg);
  color: var(--c-wrong);
  border: 1px solid var(--c-wrong);
}

/* QUIZ ACTIONS */
.quiz-actions {
  display: flex;
  justify-content: center;
  margin-top: var(--space-10);
}

/* ============================================================
   SITE FOOTER
   ============================================================ */
.site-footer {
  background: var(--c-navy);
  color: rgba(255,255,255,0.7);
  text-align: center;
  padding: var(--space-8) var(--space-6);
  font-size: 0.875rem;
  transition: background var(--transition-base);
}

[data-theme="dark"] .site-footer {
  background: var(--c-surface);
  border-top: 1px solid var(--c-border);
}

/* ============================================================
   RESPONSIVE — TABLET (< 1024px)
   ============================================================ */
@media (max-width: 1023px) {
  .hero {
    grid-template-columns: 1fr;
    padding-top: var(--space-16);
  }

  .hero-card {
    max-width: 400px;
  }

  .focus-list {
    grid-template-columns: repeat(2, 1fr);
  }

  .concept-layout {
    grid-template-columns: 1fr 1fr;
  }

  .concept-card.large {
    grid-column: 1 / -1;
  }

  .conversion-grid {
    grid-template-columns: repeat(2, 1fr);
  }

  .tool-grid {
    grid-template-columns: repeat(2, 1fr);
  }

  .tool-grid > article:last-child {
    grid-column: 1 / -1;
  }

  .worked-example {
    grid-template-columns: 1fr;
  }

  .quiz-options {
    grid-template-columns: 1fr;
  }
}

/* ============================================================
   RESPONSIVE — MOBILE (< 640px)
   ============================================================ */
@media (max-width: 639px) {
  :root {
    --nav-height: 56px;
  }

  .nav-actions {
    gap: var(--space-1);
  }

  .nav-link {
    display: none;
  }

  .brand span {
    display: none;
  }

  .hero {
    padding-top: var(--space-12);
    padding-bottom: var(--space-10);
  }

  .focus-list {
    grid-template-columns: 1fr;
  }

  .concept-layout {
    grid-template-columns: 1fr;
  }

  .concept-card.large {
    grid-column: 1;
  }

  .conversion-grid {
    grid-template-columns: 1fr;
  }

  .tool-grid {
    grid-template-columns: 1fr;
  }

  .tool-grid > article:last-child {
    grid-column: 1;
  }

  .converter-row {
    flex-direction: column;
  }

  .converter-row select,
  .converter-row input,
  .converter-row .primary-button {
    width: 100%;
  }

  .quiz-intro {
    flex-direction: column;
  }

  .score-card {
    width: 100%;
  }

  .quiz-item {
    padding: var(--space-5) var(--space-5);
  }

  .quiz-options {
    grid-template-columns: 1fr;
  }

  .auth-card {
    padding: var(--space-8) var(--space-5);
  }
}

/* ============================================================
   FOCUS VISIBLE (Accessibility)
   ============================================================ */
:focus-visible {
  outline: 2px solid var(--c-orange);
  outline-offset: 2px;
}

/* ============================================================
   SELECTION COLOUR
   ============================================================ */
::selection {
  background: rgba(244,162,97,0.3);
  color: var(--c-text);
}