/* ============================================================
   CSS Custom Properties — Brighterly Design System
   ============================================================ */
:root {
  --color-indigo-50:  #eef2ff;
  --color-indigo-100: #e0e7ff;
  --color-indigo-200: #c7d2fe;
  --color-indigo-300: #a5b4fc;
  --color-indigo-400: #818cf8;
  --color-indigo-500: #6366f1;
  --color-amber-400:  #fbbf24;
  --color-blue-500:   #3b82f6;
  --color-white:      #ffffff;
  --color-gray-text:  #1e1b4b;
  --color-gray-muted: #6b7280;

  --radius-card:  24px;
  --radius-input: 12px;
  --radius-pill:  9999px;

  --transition-fast:   0.15s ease;
  --transition-normal: 0.2s ease;
  --transition-reveal: 0.35s ease;
}

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

html {
  font-size: 16px;
  scroll-behavior: smooth;
}

body {
  font-family: 'Rubik', sans-serif;
  background-color: var(--color-indigo-50);
  color: var(--color-gray-text);
  line-height: 1.5;
  -webkit-font-smoothing: antialiased;
}

/* ============================================================
   Typography
   ============================================================ */
h1 {
  font-weight: 700;
  font-size: 64px;
  line-height: 72px;
  color: var(--color-gray-text);
}

h2 {
  font-weight: 700;
  font-size: 48px;
  line-height: 64px;
  color: var(--color-gray-text);
  margin-bottom: 20px;
}

h3 {
  font-weight: 600;
  font-size: 32px;
  line-height: 40px;
  color: var(--color-gray-text);
  margin: 24px 0 12px;
}

h4 {
  font-weight: 600;
  font-size: 24px;
  line-height: 32px;
  color: var(--color-gray-text);
}

p {
  font-size: 18px;
  line-height: 24px;
  color: var(--color-gray-text);
  margin-bottom: 16px;
}

p:last-child {
  margin-bottom: 0;
}

ol, ul {
  padding-left: 24px;
  margin-bottom: 16px;
}

li {
  font-size: 18px;
  line-height: 28px;
  color: var(--color-gray-text);
  margin-bottom: 6px;
}

strong {
  font-weight: 600;
}

/* ============================================================
   Hero Section
   ============================================================ */
.page-hero {
  background: var(--color-white);
  border-bottom: 1px solid var(--color-indigo-200);
  padding: 64px 24px 56px;
  text-align: center;
}

.hero-badge {
  display: inline-block;
  background: var(--color-amber-400);
  color: var(--color-gray-text);
  font-weight: 500;
  font-size: 16px;
  line-height: 24px;
  padding: 6px 18px;
  border-radius: var(--radius-pill);
  margin-bottom: 24px;
}

.page-hero h1 {
  margin-bottom: 16px;
}

.hero-subtitle {
  font-size: 20px;
  line-height: 28px;
  color: var(--color-gray-muted);
  max-width: 560px;
  margin: 0 auto;
}

/* ============================================================
   Page Container
   ============================================================ */
.page-container {
  max-width: 800px;
  margin: 0 auto;
  padding: 48px 24px 80px;
}

/* ============================================================
   Calculator Card
   ============================================================ */
.calculator-card {
  background: var(--color-white);
  border: 1px solid var(--color-indigo-200);
  border-radius: var(--radius-card);
  padding: 40px;
}

/* ============================================================
   Operation Tabs
   ============================================================ */
.operation-tabs {
  display: flex;
  gap: 8px;
  background: var(--color-indigo-50);
  border-radius: 12px;
  padding: 4px;
  margin-bottom: 32px;
}

.operation-tab {
  flex: 1;
  padding: 10px 16px;
  border: none;
  border-radius: 8px;
  font-family: 'Rubik', sans-serif;
  font-weight: 500;
  font-size: 16px;
  cursor: pointer;
  transition: all var(--transition-normal);
  background: transparent;
  color: var(--color-gray-muted);
  min-height: 44px;
}

.operation-tab:hover:not(.active) {
  background: var(--color-indigo-100);
  color: var(--color-indigo-500);
}

.operation-tab.active {
  background: var(--color-indigo-500);
  color: var(--color-white);
  box-shadow: 0 2px 8px rgba(99, 102, 241, 0.25);
}

.operation-tab:focus-visible {
  outline: 3px solid var(--color-indigo-500);
  outline-offset: 2px;
}

/* ============================================================
   Inputs Row
   ============================================================ */
.inputs-row {
  display: flex;
  gap: 16px;
  align-items: center;
  margin-bottom: 28px;
}

/* Operator symbol between inputs */
.operator-symbol {
  font-size: 32px;
  font-weight: 600;
  color: var(--color-indigo-400);
  flex-shrink: 0;
  width: 40px;
  text-align: center;
  padding-top: 20px;
}

/* ============================================================
   Fraction Input Group
   ============================================================ */
.fraction-input-group {
  flex: 1;
  background: var(--color-white);
  border: 1.5px solid var(--color-indigo-200);
  border-radius: 12px;
  padding: 16px;
  transition: border-color var(--transition-normal), box-shadow var(--transition-normal);
}

.fraction-input-group:focus-within {
  border-color: var(--color-indigo-500);
  box-shadow: 0 0 0 3px rgba(99, 102, 241, 0.12);
}

.fraction-input-group.has-error {
  border-color: #ef4444;
  box-shadow: 0 0 0 3px rgba(239, 68, 68, 0.12);
}

.input-group-label {
  font-weight: 600;
  font-size: 14px;
  color: var(--color-gray-muted);
  text-transform: uppercase;
  letter-spacing: 0.06em;
  margin-bottom: 12px;
}

.fraction-fields-layout {
  display: flex;
  align-items: center;
  gap: 12px;
}

/* Whole number column */
.whole-col {
  display: flex;
  flex-direction: column;
  align-items: center;
  gap: 4px;
}

.whole-col input[type="number"] {
  width: 72px;
}

.field-hint {
  font-size: 11px;
  color: var(--color-gray-muted);
  opacity: 0.7;
  text-align: center;
}

/* Visual divider between whole and fraction */
.whole-col::after {
  content: '';
  display: none;
}

/* Fraction numerator/bar/denominator column */
.fraction-col {
  display: flex;
  flex-direction: column;
  align-items: center;
  gap: 4px;
}

.visual-bar {
  width: 100%;
  height: 3px;
  background: var(--color-indigo-300);
  border-radius: 2px;
  margin: 2px 0;
}

.fraction-col input[type="number"] {
  width: 72px;
}

/* Labels below/above inputs */
.field-sublabel {
  font-size: 11px;
  font-weight: 500;
  color: var(--color-gray-muted);
  text-align: center;
  cursor: default;
}

/* ============================================================
   Number Inputs
   ============================================================ */
input[type="number"] {
  border: 1.5px solid var(--color-indigo-200);
  border-radius: 8px;
  padding: 10px 8px;
  font-family: 'Rubik', sans-serif;
  font-size: 24px;
  font-weight: 500;
  color: var(--color-gray-text);
  width: 100%;
  text-align: center;
  background: var(--color-white);
  transition: border-color var(--transition-normal), background var(--transition-normal);
  -moz-appearance: textfield;
  min-height: 52px;
}

input[type="number"]::-webkit-outer-spin-button,
input[type="number"]::-webkit-inner-spin-button {
  -webkit-appearance: none;
  margin: 0;
}

input[type="number"]:focus {
  outline: none;
  border-color: var(--color-indigo-500);
  background: var(--color-indigo-50);
}

input[type="number"].error {
  border-color: #ef4444;
  background: #fef2f2;
}

/* ============================================================
   Error Messages
   ============================================================ */
.input-error {
  display: none;
  margin-top: 10px;
  font-size: 13px;
  font-weight: 500;
  color: #ef4444;
  line-height: 1.4;
}

.input-error.visible {
  display: block;
}

/* ============================================================
   Calculate Button
   ============================================================ */
.calculate-btn {
  background: var(--color-indigo-500);
  color: var(--color-white);
  border: none;
  border-radius: 12px;
  padding: 16px 40px;
  font-family: 'Rubik', sans-serif;
  font-weight: 600;
  font-size: 18px;
  cursor: pointer;
  width: 100%;
  transition: transform var(--transition-fast), background var(--transition-normal);
  min-height: 56px;
}

.calculate-btn:hover {
  background: var(--color-indigo-400);
  transform: scale(1.02);
}

.calculate-btn:active {
  transform: scale(0.98);
}

.calculate-btn:focus-visible {
  outline: 3px solid var(--color-indigo-500);
  outline-offset: 3px;
}

/* ============================================================
   Result Card
   ============================================================ */
.result-card {
  background: var(--color-white);
  border-radius: 20px;
  border: 1px solid var(--color-indigo-200);
  border-left: 5px solid var(--color-amber-400);
  padding: 32px;
  margin-top: 24px;
  opacity: 0;
  transform: translateY(16px);
  transition: opacity var(--transition-reveal), transform var(--transition-reveal);
  pointer-events: none;
}

.result-card.visible {
  opacity: 1;
  transform: translateY(0);
  pointer-events: auto;
}

.result-label {
  font-size: 12px;
  font-weight: 600;
  text-transform: uppercase;
  letter-spacing: 0.08em;
  color: var(--color-amber-400);
  margin-bottom: 16px;
  text-align: center;
}

.result-fraction {
  font-size: 48px;
  font-weight: 700;
  color: var(--color-indigo-500);
  text-align: center;
  display: flex;
  align-items: center;
  justify-content: center;
  gap: 8px;
  flex-wrap: wrap;
  min-height: 64px;
}

.result-decimal {
  font-size: 18px;
  color: var(--color-gray-muted);
  text-align: center;
  margin-top: 8px;
}

/* ============================================================
   Fraction Display (stacked visual)
   ============================================================ */
.fraction-display {
  display: inline-flex;
  flex-direction: column;
  align-items: center;
  vertical-align: middle;
  margin: 0 4px;
  line-height: 1.2;
}

.fraction-num {
  font-weight: 600;
}

.fraction-bar {
  width: 100%;
  min-width: 18px;
  height: 2px;
  background: currentColor;
  margin: 3px 0;
  display: block;
}

.fraction-den {
  font-weight: 600;
}

/* Whole number display inline */
.whole-number {
  display: inline;
  vertical-align: middle;
  font-weight: 600;
}

/* Mixed number display */
.mixed-display {
  display: inline-flex;
  align-items: center;
  gap: 4px;
  vertical-align: middle;
}

/* Inline fraction for prose text */
.inline-fraction {
  display: inline-flex;
  align-items: center;
  vertical-align: middle;
  font-size: 0.9em;
}

/* ============================================================
   Steps Container
   ============================================================ */
.steps-container {
  margin-top: 32px;
}

.steps-header {
  font-weight: 700;
  font-size: 20px;
  color: var(--color-gray-text);
  margin-bottom: 20px;
  padding-bottom: 12px;
  border-bottom: 2px solid var(--color-indigo-100);
}

.step-item {
  display: flex;
  gap: 16px;
  margin-bottom: 20px;
  align-items: flex-start;
}

.step-badge {
  width: 32px;
  height: 32px;
  border-radius: 50%;
  background: var(--color-indigo-500);
  color: var(--color-white);
  font-family: 'Rubik', sans-serif;
  font-weight: 600;
  font-size: 14px;
  display: flex;
  align-items: center;
  justify-content: center;
  flex-shrink: 0;
  margin-top: 4px;
}

.step-content {
  background: var(--color-indigo-50);
  border-radius: 12px;
  padding: 16px 20px;
  flex: 1;
}

.step-title {
  font-weight: 600;
  font-size: 16px;
  color: var(--color-blue-500);
  margin-bottom: 8px;
}

.step-math {
  font-size: 18px;
  color: var(--color-gray-text);
  line-height: 2;
  display: flex;
  flex-wrap: wrap;
  align-items: center;
  gap: 8px;
}

.decimal-result {
  font-weight: 600;
  color: var(--color-indigo-500);
}

/* ============================================================
   Curriculum Sections
   ============================================================ */
.curriculum-section {
  margin-top: 64px;
  padding-top: 48px;
  border-top: 2px solid var(--color-indigo-100);
}

.curriculum-section:first-of-type {
  border-top: none;
  margin-top: 64px;
  padding-top: 0;
}

.curriculum-section h2 {
  margin-bottom: 16px;
}

.curriculum-section h3 {
  font-size: 22px;
  line-height: 30px;
  margin: 20px 0 8px;
}

/* ============================================================
   Example Box
   ============================================================ */
.example-box {
  background: var(--color-indigo-50);
  border: 1px solid var(--color-indigo-200);
  border-radius: 16px;
  padding: 24px;
  margin: 20px 0;
}

.example-label {
  font-family: 'Rubik', sans-serif;
  font-weight: 600;
  font-size: 12px;
  text-transform: uppercase;
  letter-spacing: 0.08em;
  color: var(--color-indigo-500);
  margin-bottom: 12px;
}

.worked-example {
  display: flex;
  flex-direction: column;
  gap: 10px;
}

.we-line {
  font-size: 18px;
  color: var(--color-gray-text);
  line-height: 2;
  display: flex;
  flex-wrap: wrap;
  align-items: center;
  gap: 8px;
}

/* Vocab grid in "What is a fraction?" */
.vocab-grid {
  display: grid;
  grid-template-columns: 1fr 1fr;
  gap: 16px;
}

.vocab-item {
  display: flex;
  flex-direction: column;
  gap: 6px;
}

.vocab-term {
  font-weight: 600;
  font-size: 15px;
  color: var(--color-indigo-500);
}

.vocab-def {
  font-size: 15px;
  color: var(--color-gray-text);
  line-height: 1.8;
  display: flex;
  flex-wrap: wrap;
  align-items: center;
  gap: 4px;
}

/* Method steps list */
.method-steps {
  padding-left: 20px;
  margin: 16px 0;
}

.method-steps li {
  font-size: 17px;
  line-height: 26px;
  margin-bottom: 8px;
}

/* ============================================================
   Mistakes Section
   ============================================================ */
.mistakes-grid {
  display: grid;
  grid-template-columns: 1fr 1fr;
  gap: 20px;
  margin-top: 24px;
}

.mistake-card {
  background: var(--color-white);
  border: 1px solid var(--color-indigo-200);
  border-radius: 16px;
  padding: 20px;
}

.mistake-label {
  font-size: 11px;
  font-weight: 600;
  text-transform: uppercase;
  letter-spacing: 0.08em;
  color: var(--color-indigo-400);
  margin-bottom: 8px;
}

.mistake-title {
  font-weight: 600;
  font-size: 16px;
  color: var(--color-gray-text);
  margin-bottom: 12px;
  line-height: 1.4;
}

.mistake-wrong,
.mistake-fix {
  font-size: 15px;
  line-height: 2;
  display: flex;
  flex-wrap: wrap;
  align-items: center;
  gap: 6px;
  margin-bottom: 8px;
}

.wrong-badge {
  font-size: 12px;
  font-weight: 600;
  color: #ef4444;
  background: #fef2f2;
  border-radius: 6px;
  padding: 2px 8px;
  white-space: nowrap;
}

.fix-badge {
  font-size: 12px;
  font-weight: 600;
  color: #16a34a;
  background: #f0fdf4;
  border-radius: 6px;
  padding: 2px 8px;
  white-space: nowrap;
}

/* ============================================================
   FAQ Section
   ============================================================ */
.faq-section {
  margin-top: 64px;
  padding-top: 48px;
  border-top: 2px solid var(--color-indigo-100);
}

.faq-section h2 {
  margin-bottom: 24px;
}

.faq-item {
  border: 1px solid var(--color-indigo-200);
  border-radius: 12px;
  margin-bottom: 12px;
  overflow: hidden;
  background: var(--color-white);
}

.faq-question {
  padding: 20px 24px;
  font-family: 'Rubik', sans-serif;
  font-weight: 600;
  font-size: 18px;
  color: var(--color-gray-text);
  cursor: pointer;
  list-style: none;
  display: flex;
  justify-content: space-between;
  align-items: center;
  gap: 16px;
  min-height: 44px;
  transition: background var(--transition-normal);
}

.faq-question::-webkit-details-marker {
  display: none;
}

.faq-question::after {
  content: '+';
  font-size: 24px;
  font-weight: 400;
  color: var(--color-indigo-500);
  transition: transform var(--transition-normal);
  flex-shrink: 0;
  line-height: 1;
}

details[open] > .faq-question {
  background: var(--color-indigo-50);
}

details[open] > .faq-question::after {
  transform: rotate(45deg);
}

.faq-question:hover {
  background: var(--color-indigo-50);
}

.faq-answer {
  padding: 0 24px 20px;
}

.faq-answer p {
  font-size: 16px;
  line-height: 24px;
  color: var(--color-gray-muted);
  margin-bottom: 12px;
}

.faq-answer ol,
.faq-answer ul {
  padding-left: 22px;
  margin-bottom: 12px;
}

.faq-answer li {
  font-size: 16px;
  line-height: 24px;
  color: var(--color-gray-muted);
  margin-bottom: 4px;
}

/* ============================================================
   Author Card
   ============================================================ */
.author-card {
  display: flex;
  gap: 20px;
  background: var(--color-indigo-50);
  border-radius: 16px;
  padding: 24px;
  margin-top: 48px;
  align-items: flex-start;
  border: 1px solid var(--color-indigo-200);
}

.author-avatar {
  width: 56px;
  height: 56px;
  border-radius: 50%;
  background: var(--color-indigo-500);
  color: var(--color-white);
  display: flex;
  align-items: center;
  justify-content: center;
  font-family: 'Rubik', sans-serif;
  font-weight: 700;
  font-size: 18px;
  flex-shrink: 0;
}

.author-name {
  font-weight: 700;
  font-size: 18px;
  color: var(--color-gray-text);
  margin-bottom: 2px;
}

.author-role {
  font-size: 14px;
  color: var(--color-indigo-500);
  font-weight: 500;
  margin-bottom: 8px;
}

.author-bio {
  font-size: 15px;
  line-height: 22px;
  color: var(--color-gray-muted);
  margin-bottom: 0;
}

/* ============================================================
   Responsive — Tablet (≤768px)
   ============================================================ */
@media (max-width: 768px) {
  h1 {
    font-size: 36px;
    line-height: 44px;
  }

  h2 {
    font-size: 28px;
    line-height: 36px;
  }

  h3 {
    font-size: 24px;
    line-height: 32px;
  }

  h4 {
    font-size: 20px;
    line-height: 28px;
  }

  p, li {
    font-size: 16px;
    line-height: 22px;
  }

  .page-hero {
    padding: 40px 20px 36px;
  }

  .page-hero h1 {
    font-size: 36px;
    line-height: 44px;
  }

  .hero-subtitle {
    font-size: 17px;
    line-height: 25px;
  }

  .page-container {
    padding: 32px 16px 60px;
  }

  .calculator-card {
    padding: 24px;
  }

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

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

/* ============================================================
   Responsive — Mobile (≤640px)
   ============================================================ */
@media (max-width: 640px) {
  .operation-tabs {
    gap: 4px;
  }

  .operation-tab {
    padding: 10px 8px;
    font-size: 14px;
  }

  /* Stack inputs vertically on mobile */
  .inputs-row {
    flex-direction: column;
    gap: 16px;
  }

  .operator-symbol {
    padding-top: 0;
    font-size: 28px;
  }

  .fraction-input-group {
    width: 100%;
  }

  .whole-col input[type="number"],
  .fraction-col input[type="number"] {
    width: 60px;
    font-size: 20px;
  }

  .result-fraction {
    font-size: 36px;
  }

  .faq-question {
    font-size: 16px;
    padding: 16px 18px;
  }

  .faq-answer {
    padding: 0 18px 16px;
  }

  .author-card {
    flex-direction: column;
  }

  .we-line {
    font-size: 16px;
  }

  .step-math {
    font-size: 16px;
  }
}

/* ============================================================
   Focus & Accessibility
   ============================================================ */
:focus-visible {
  outline: 3px solid var(--color-indigo-500);
  outline-offset: 2px;
  border-radius: 4px;
}

/* Skip hidden elements from layout flow until revealed */
.result-card[aria-live] {
  display: block;
}