:root {
  --bg: #f6f7fb;
  --surface: #ffffff;
  --surface-soft: #f0f5ff;
  --ink: #182131;
  --muted: #657084;
  --line: #d9dfeb;
  --primary: #1f6f8b;
  --primary-dark: #145673;
  --accent: #d85f45;
  --success: #257a4e;
  --warning: #b57913;
  --shadow: 0 16px 36px rgba(24, 33, 49, 0.08);
}

* {
  box-sizing: border-box;
}

body {
  margin: 0;
  min-height: 100vh;
  color: var(--ink);
  background: var(--bg);
  font-family: Inter, ui-sans-serif, system-ui, -apple-system, BlinkMacSystemFont, "Segoe UI", sans-serif;
}

button,
input,
select {
  font: inherit;
}

button {
  min-height: 42px;
  border: 0;
  border-radius: 8px;
  padding: 0 16px;
  color: #ffffff;
  background: var(--primary);
  font-weight: 800;
  cursor: pointer;
}

button:hover {
  background: var(--primary-dark);
}

button:disabled {
  cursor: not-allowed;
  opacity: 0.45;
}

button:focus-visible,
input:focus-visible,
select:focus-visible {
  outline: 3px solid rgba(31, 111, 139, 0.26);
  outline-offset: 2px;
}

.app-shell {
  width: min(1380px, 100%);
  margin: 0 auto;
  padding: 24px;
}

.app-header {
  display: flex;
  align-items: end;
  justify-content: space-between;
  gap: 18px;
  margin-bottom: 18px;
}

.brand-block h1 {
  margin: 8px 0 0;
  font-size: clamp(2.1rem, 5vw, 4.5rem);
  line-height: 0.95;
  letter-spacing: 0;
}

.grade-badge,
.standard-chip {
  display: inline-flex;
  align-items: center;
  justify-content: center;
  min-height: 30px;
  border-radius: 999px;
  padding: 4px 10px;
  color: var(--primary);
  background: #e0f2f7;
  font-size: 0.82rem;
  font-weight: 900;
}

.header-actions {
  display: flex;
  align-items: end;
  gap: 12px;
}

.grade-picker,
.test-panel label {
  display: grid;
  gap: 6px;
  color: var(--muted);
  font-size: 0.88rem;
  font-weight: 850;
}

select,
input {
  width: 100%;
  min-height: 42px;
  border: 1px solid var(--line);
  border-radius: 8px;
  padding: 0 12px;
  color: var(--ink);
  background: var(--surface);
}

.view-tabs {
  display: grid;
  grid-template-columns: 1fr 1fr;
  gap: 4px;
  padding: 4px;
  border: 1px solid var(--line);
  border-radius: 8px;
  background: var(--surface);
}

.view-tab {
  min-width: 116px;
  color: var(--ink);
  background: transparent;
}

.view-tab:hover {
  color: var(--ink);
  background: #eef2f7;
}

.view-tab.active {
  color: #ffffff;
  background: var(--ink);
}

.view {
  display: none;
}

.view.active {
  display: grid;
  grid-template-columns: 318px minmax(0, 1fr);
  gap: 18px;
  align-items: start;
}

.side-panel,
.lesson-panel,
.quiz-panel,
.results-panel {
  border: 1px solid var(--line);
  border-radius: 8px;
  background: var(--surface);
  box-shadow: var(--shadow);
}

.side-panel {
  position: sticky;
  top: 18px;
  padding: 16px;
}

.panel-title,
.lesson-heading,
.quiz-heading,
.quiz-meta,
.quiz-actions {
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 12px;
}

.panel-title {
  margin-bottom: 14px;
}

.panel-title h2,
.lesson-heading h2,
.quiz-heading h2,
.results-panel h2 {
  margin: 0;
  line-height: 1.1;
  letter-spacing: 0;
}

.panel-title h2 {
  font-size: 1.08rem;
}

.panel-title span,
.quiz-meta span {
  color: var(--muted);
  font-size: 0.86rem;
  font-weight: 850;
}

.lesson-list {
  display: grid;
  gap: 8px;
}

.lesson-button {
  display: grid;
  grid-template-columns: 28px minmax(0, 1fr);
  gap: 10px;
  align-items: center;
  width: 100%;
  min-height: 58px;
  padding: 10px;
  border: 1px solid var(--line);
  color: var(--ink);
  background: var(--surface);
  text-align: left;
}

.lesson-button:hover {
  color: var(--ink);
  background: #f5f7fb;
}

.lesson-button.active {
  border-color: rgba(31, 111, 139, 0.55);
  background: #eef9fb;
}

.lesson-number {
  display: grid;
  place-items: center;
  width: 28px;
  height: 28px;
  border-radius: 8px;
  color: #ffffff;
  background: var(--primary);
  font-size: 0.82rem;
  font-weight: 900;
}

.lesson-name {
  overflow-wrap: anywhere;
  font-size: 0.93rem;
  font-weight: 850;
  line-height: 1.2;
}

.content-panel {
  display: grid;
  gap: 18px;
}

.lesson-panel,
.quiz-panel,
.results-panel {
  padding: 22px;
}

.lesson-heading,
.quiz-heading {
  margin-bottom: 18px;
}

.lesson-heading h2,
.quiz-heading h2,
.results-panel h2 {
  font-size: clamp(1.45rem, 2vw, 2.1rem);
}

.eyebrow {
  margin: 0 0 6px;
  color: var(--accent);
  font-size: 0.76rem;
  font-weight: 900;
  letter-spacing: 0;
  text-transform: uppercase;
}

.math-visual {
  display: grid;
  place-items: center;
  min-height: 170px;
  margin-bottom: 20px;
  border: 1px solid var(--line);
  border-radius: 8px;
  background: #fbfcff;
  overflow: hidden;
  padding: 18px;
}

.math-visual.compact {
  min-height: 122px;
}

.picture-strip {
  display: grid;
  grid-template-columns: repeat(3, minmax(0, 1fr));
  gap: 12px;
  margin: -6px 0 20px;
}

.picture-card,
.solution-panel,
.grade-tile {
  border: 1px solid var(--line);
  border-radius: 8px;
  background: #ffffff;
}

.picture-card {
  display: grid;
  gap: 10px;
  min-height: 154px;
  padding: 12px;
}

.picture-card strong,
.solution-panel strong,
.grade-tile strong,
.result-status {
  font-weight: 900;
}

.picture-card p {
  margin: 0;
  color: var(--muted);
  font-size: 0.9rem;
  line-height: 1.35;
}

.mini-art,
.solution-art {
  display: grid;
  place-items: center;
  min-height: 82px;
  border-radius: 8px;
  background: #f4f8fb;
  overflow: hidden;
  padding: 10px;
}

.solution-panel {
  display: grid;
  grid-template-columns: minmax(170px, 0.55fr) minmax(0, 1fr);
  gap: 14px;
  margin-top: 12px;
  padding: 14px;
}

.solution-panel[hidden] {
  display: none;
}

.solution-copy {
  display: grid;
  gap: 8px;
}

.solution-copy p {
  margin: 0;
  color: #384253;
  line-height: 1.5;
}

.solution-copy ol {
  display: grid;
  gap: 6px;
  margin: 0;
  padding-left: 20px;
}

.solution-copy li {
  line-height: 1.42;
}

.lesson-section {
  margin-top: 18px;
}

.lesson-section h3 {
  margin: 0 0 8px;
  font-size: 1rem;
}

.lesson-section p,
.lesson-section li,
.example-card p,
.feedback,
.result-item p {
  line-height: 1.5;
}

.lesson-section p {
  margin: 0;
  color: #384253;
}

.lesson-section ul {
  display: grid;
  gap: 8px;
  margin: 0;
  padding-left: 20px;
}

.lesson-grid {
  display: grid;
  grid-template-columns: repeat(2, minmax(0, 1fr));
  gap: 18px;
}

.example-list {
  display: grid;
  grid-template-columns: repeat(2, minmax(0, 1fr));
  gap: 12px;
}

.example-card {
  border: 1px solid var(--line);
  border-radius: 8px;
  padding: 14px;
  background: #fbfcff;
}

.example-card strong {
  display: block;
  margin-bottom: 6px;
}

.example-card p {
  margin: 0;
  color: #3a4455;
}

.quiz-meta {
  min-height: 32px;
  margin-bottom: 14px;
  padding: 8px 10px;
  border-radius: 8px;
  background: #f5f7fb;
}

.question-text {
  margin: 0 0 14px;
  color: var(--ink);
  font-size: 1.16rem;
  font-weight: 850;
  line-height: 1.42;
}

.answer-form {
  display: grid;
  grid-template-columns: minmax(0, 1fr) auto;
  gap: 10px;
}

.choice-grid {
  display: grid;
  grid-template-columns: repeat(2, minmax(0, 1fr));
  gap: 10px;
}

.choice-button {
  min-height: 48px;
  border: 1px solid var(--line);
  color: var(--ink);
  background: #ffffff;
}

.choice-button:hover,
.choice-button.selected {
  color: var(--ink);
  background: var(--surface-soft);
}

.feedback {
  min-height: 28px;
  margin: 12px 0 0;
  color: var(--muted);
  font-weight: 760;
}

.feedback.correct {
  color: var(--success);
}

.feedback.incorrect {
  color: var(--accent);
}

.secondary-button {
  border: 1px solid var(--line);
  color: var(--ink);
  background: var(--surface);
}

.secondary-button:hover {
  color: var(--ink);
  background: #f1f4f8;
}

.test-panel {
  display: grid;
  gap: 14px;
}

.score-meter {
  height: 16px;
  margin: 14px 0;
  border-radius: 999px;
  background: #edf1f6;
  overflow: hidden;
}

.score-meter span {
  display: block;
  width: 0%;
  height: 100%;
  background: var(--primary);
}

.grade-summary {
  display: grid;
  grid-template-columns: repeat(4, minmax(0, 1fr));
  gap: 10px;
  margin: 14px 0;
}

.grade-tile {
  min-height: 78px;
  padding: 12px;
}

.grade-tile span {
  display: block;
  color: var(--muted);
  font-size: 0.78rem;
  font-weight: 850;
}

.grade-tile strong {
  display: block;
  margin-top: 5px;
  color: var(--ink);
  font-size: 1.25rem;
}

.result-list {
  display: grid;
  gap: 10px;
}

.result-item {
  display: grid;
  grid-template-columns: 30px minmax(0, 1fr);
  gap: 10px;
  padding: 12px;
  border-radius: 8px;
  background: #f7f9fc;
}

.result-mark {
  display: grid;
  place-items: center;
  width: 30px;
  height: 30px;
  border-radius: 999px;
  color: #ffffff;
  background: var(--accent);
  font-weight: 900;
}

.result-item.correct .result-mark {
  background: var(--success);
}

.result-item p {
  margin: 0;
}

.result-status {
  display: inline-flex;
  margin: 0 0 5px;
  padding: 3px 8px;
  border-radius: 999px;
  color: #ffffff;
  background: var(--accent);
  font-size: 0.78rem;
}

.result-item.correct .result-status {
  background: var(--success);
}

.place-model,
.ratio-model,
.tiles-model,
.stats-model {
  display: grid;
  width: min(620px, 100%);
  gap: 10px;
}

.place-model {
  grid-template-columns: repeat(4, minmax(70px, 1fr));
}

.place-cell,
.ratio-cell,
.stat-cell {
  min-height: 86px;
  border: 1px solid var(--line);
  border-radius: 8px;
  background: #ffffff;
  padding: 10px;
}

.place-cell span,
.ratio-cell span,
.stat-cell span {
  display: block;
  color: var(--muted);
  font-size: 0.78rem;
  font-weight: 850;
}

.place-cell strong,
.ratio-cell strong,
.stat-cell strong {
  display: block;
  margin-top: 8px;
  color: var(--primary);
  font-size: 1.7rem;
}

.fraction-model {
  display: grid;
  width: min(600px, 100%);
  gap: 14px;
}

.fraction-bar {
  display: grid;
  min-height: 42px;
  border: 2px solid var(--ink);
  border-radius: 8px;
  overflow: hidden;
  background: #ffffff;
}

.fraction-piece {
  border-right: 1px solid rgba(24, 33, 49, 0.35);
}

.fraction-piece:last-child {
  border-right: 0;
}

.fraction-piece.fill {
  background: var(--accent);
}

.decimal-grid {
  display: grid;
  grid-template-columns: repeat(10, 1fr);
  width: min(300px, 78vw);
  aspect-ratio: 1;
  border: 2px solid var(--ink);
  background: #ffffff;
}

.decimal-square {
  border: 1px solid rgba(24, 33, 49, 0.18);
}

.decimal-square.fill {
  background: var(--primary);
}

.tiles-model {
  grid-template-columns: repeat(4, minmax(54px, 1fr));
  width: min(360px, 78vw);
}

.tile {
  aspect-ratio: 1;
  border-radius: 8px;
  background: #dfeaf8;
  border: 1px solid #b9cce4;
}

.grid-model {
  display: grid;
  grid-template-columns: repeat(6, 1fr);
  width: min(320px, 76vw);
  aspect-ratio: 1;
  border-left: 3px solid var(--ink);
  border-bottom: 3px solid var(--ink);
  background:
    linear-gradient(90deg, rgba(31, 111, 139, 0.22) 1px, transparent 1px),
    linear-gradient(0deg, rgba(31, 111, 139, 0.22) 1px, transparent 1px),
    #ffffff;
  background-size: calc(100% / 6) calc(100% / 6);
}

.grid-dot {
  align-self: end;
  justify-self: start;
  width: 17px;
  height: 17px;
  margin: 0 0 -9px -9px;
  border: 3px solid #ffffff;
  border-radius: 999px;
  background: var(--accent);
}

.ratio-model {
  grid-template-columns: repeat(3, minmax(82px, 1fr));
}

.stats-model {
  grid-template-columns: repeat(6, 1fr);
  align-items: end;
  min-height: 140px;
}

.stat-cell {
  display: grid;
  align-items: end;
  min-height: 50px;
  background: #eef6f8;
}

.mini-number-line {
  position: relative;
  width: 100%;
  height: 44px;
}

.mini-number-line::before {
  content: "";
  position: absolute;
  left: 6px;
  right: 6px;
  top: 21px;
  height: 3px;
  background: var(--ink);
}

.mini-tick {
  position: absolute;
  top: 12px;
  width: 3px;
  height: 21px;
  background: var(--ink);
}

.mini-dot {
  position: absolute;
  top: 13px;
  width: 18px;
  height: 18px;
  border: 3px solid #ffffff;
  border-radius: 999px;
  background: var(--accent);
  box-shadow: 0 3px 12px rgba(216, 95, 69, 0.22);
}

.mini-equation {
  display: flex;
  align-items: center;
  justify-content: center;
  gap: 8px;
  width: 100%;
  color: var(--primary);
  font-size: 1.2rem;
  font-weight: 900;
}

.mini-equation span {
  display: grid;
  place-items: center;
  min-width: 42px;
  min-height: 42px;
  border: 1px solid var(--line);
  border-radius: 8px;
  background: #ffffff;
}

.sr-only {
  position: absolute;
  width: 1px;
  height: 1px;
  padding: 0;
  margin: -1px;
  overflow: hidden;
  clip: rect(0, 0, 0, 0);
  white-space: nowrap;
  border: 0;
}

@media (max-width: 980px) {
  .app-header,
  .header-actions {
    align-items: stretch;
    flex-direction: column;
  }

  .view.active {
    grid-template-columns: 1fr;
  }

  .side-panel {
    position: static;
  }

  .lesson-list {
    grid-template-columns: repeat(2, minmax(0, 1fr));
  }

  .example-list,
  .picture-strip,
  .grade-summary,
  .lesson-grid {
    grid-template-columns: 1fr;
  }

  .solution-panel {
    grid-template-columns: 1fr;
  }
}

@media (max-width: 640px) {
  .app-shell {
    padding: 14px;
  }

  .lesson-panel,
  .quiz-panel,
  .results-panel {
    padding: 16px;
  }

  .view-tabs,
  .lesson-list,
  .answer-form,
  .choice-grid,
  .place-model,
  .ratio-model {
    grid-template-columns: 1fr;
  }
}