:root {
  --bg: #f4f6fb;
  --surface: #ffffff;
  --surface-2: #f8f9fd;
  --text: #172033;
  --muted: #5f6b80;
  --line: #d9deea;
  --primary: #3157d5;
  --primary-strong: #2443aa;
  --primary-soft: #e9edff;
  --success: #16784a;
  --success-soft: #e8f7ef;
  --danger: #aa3d45;
  --danger-soft: #fff0f1;
  --warning: #916400;
  --warning-soft: #fff8df;
  --focus: #f59e0b;
  --shadow: 0 12px 34px rgba(34, 51, 84, .10);
  --radius: 22px;
  --radius-sm: 13px;
  --font: Inter, ui-rounded, "SF Pro Rounded", "Segoe UI", system-ui, sans-serif;
  --display-font: Inter, ui-rounded, "SF Pro Rounded", "Segoe UI", system-ui, sans-serif;
  --motion: 280ms;
}

* { box-sizing: border-box; }
.sr-only { position: absolute !important; width: 1px !important; height: 1px !important; padding: 0 !important; margin: -1px !important; overflow: hidden !important; clip: rect(0, 0, 0, 0) !important; white-space: nowrap !important; border: 0 !important; }
html { scroll-behavior: smooth; }
body {
  margin: 0;
  min-width: 320px;
  min-height: 100vh;
  color: var(--text);
  background: var(--bg);
  font-family: var(--font);
  font-size: clamp(16px, 1.2vw, 18px);
  line-height: 1.45;
  padding: max(0px, env(safe-area-inset-top)) max(0px, env(safe-area-inset-right)) max(18px, env(safe-area-inset-bottom)) max(0px, env(safe-area-inset-left));
  transition: background var(--motion), color var(--motion);
}
button, input, select { font: inherit; }
button { color: inherit; }
button, select, input { min-height: 44px; }
button:focus-visible, input:focus-visible, select:focus-visible, [tabindex]:focus-visible {
  outline: 4px solid color-mix(in srgb, var(--focus) 55%, transparent);
  outline-offset: 3px;
}
button:disabled { opacity: .48; cursor: not-allowed; }

.skip-link {
  position: fixed;
  left: 12px;
  top: -80px;
  z-index: 999;
  padding: 10px 14px;
  border-radius: 10px;
  background: var(--text);
  color: var(--surface);
}
.skip-link:focus { top: 12px; }

.app-header {
  width: min(1480px, 100%);
  margin: 0 auto;
  padding: clamp(22px, 4vw, 48px) clamp(18px, 4vw, 52px) 18px;
  display: flex;
  align-items: flex-end;
  justify-content: space-between;
  gap: 24px;
}
h1, h2, h3, p { margin-top: 0; }
h1 {
  margin-bottom: 6px;
  font-family: var(--display-font);
  font-size: clamp(2rem, 4vw, 3.6rem);
  line-height: 1;
  letter-spacing: -.045em;
}
h2 { margin-bottom: 0; font-size: clamp(1.35rem, 2.4vw, 2rem); letter-spacing: -.025em; }
h3 { margin-bottom: 0; font-size: clamp(1.15rem, 2vw, 1.55rem); }
.subtitle { margin: 0; color: var(--muted); }
.eyebrow, .section-kicker {
  margin-bottom: 4px;
  color: var(--primary);
  font-size: .76rem;
  font-weight: 850;
  letter-spacing: .12em;
  text-transform: uppercase;
}
.header-actions { display: flex; gap: 10px; flex-wrap: wrap; justify-content: flex-end; }

.app-shell {
  width: min(1480px, calc(100% - 24px));
  margin: 0 auto;
  display: grid;
  gap: 18px;
}
.problem-card, .workspace, .teacher-panel {
  border: 1px solid var(--line);
  border-radius: var(--radius);
  background: var(--surface);
  box-shadow: var(--shadow);
}
.problem-card { padding: clamp(18px, 3vw, 32px); }
.section-heading-row, .problem-summary-row, .workspace-topbar, .teacher-panel-header, .decimal-demo-header, .instruction-header {
  display: flex;
  justify-content: space-between;
  align-items: center;
  gap: 16px;
}

.problem-form {
  display: grid;
  grid-template-columns: minmax(130px, 1fr) auto minmax(130px, 1fr) auto;
  gap: 14px;
  align-items: end;
  margin-top: 22px;
}
.problem-form label, .settings-grid label { display: grid; gap: 6px; font-weight: 760; }
.problem-form small { color: var(--muted); font-weight: 500; font-size: .78rem; }
.problem-form input, .settings-grid select, .compact-select select {
  width: 100%;
  border: 2px solid var(--line);
  border-radius: 13px;
  background: var(--surface-2);
  color: var(--text);
  padding: 10px 13px;
}
.problem-form input {
  min-height: 58px;
  font-family: var(--display-font);
  font-size: clamp(1.4rem, 3vw, 2rem);
  font-weight: 850;
  font-variant-numeric: tabular-nums;
}
.operator { align-self: center; padding-top: 22px; font-size: 2.1rem; font-weight: 850; }
.button {
  border: 0;
  border-radius: 13px;
  padding: 11px 17px;
  font-weight: 820;
  cursor: pointer;
  transition: transform var(--motion), background var(--motion), box-shadow var(--motion);
}
.button:hover:not(:disabled) { transform: translateY(-1px); }
.button.primary { background: var(--primary); color: #fff; box-shadow: 0 8px 18px color-mix(in srgb, var(--primary) 24%, transparent); }
.button.primary:hover:not(:disabled) { background: var(--primary-strong); }
.button.secondary { background: var(--primary-soft); color: var(--primary-strong); }
.button.danger-outline { border: 2px solid var(--danger); background: transparent; color: var(--danger); }
.start-button { min-height: 58px; white-space: nowrap; }
.icon-button { display: inline-flex; align-items: center; gap: 8px; background: var(--surface); border: 1px solid var(--line); }
.icon-only { border: 0; background: transparent; font-size: 1.8rem; cursor: pointer; width: 48px; height: 48px; }
.text-button {
  border: 0;
  background: transparent;
  color: var(--primary);
  font-weight: 820;
  text-decoration: underline;
  text-decoration-thickness: 2px;
  text-underline-offset: 4px;
  cursor: pointer;
  padding: 6px 2px;
}
.message { min-height: 24px; margin-top: 10px; font-weight: 700; }
.message.error { color: var(--danger); }
.message.success { color: var(--success); }

.problem-summary-row { margin-top: 20px; padding-top: 18px; border-top: 1px solid var(--line); align-items: flex-end; }
.equation-display {
  min-height: 64px;
  display: flex;
  align-items: center;
  flex-wrap: wrap;
  gap: 8px;
  font-family: var(--display-font);
  font-size: clamp(1.6rem, 4vw, 2.8rem);
  font-weight: 900;
  font-variant-numeric: tabular-nums;
}
.vocab-number { position: relative; display: inline-grid; justify-items: center; }
.vocab-number small { font-family: var(--font); font-size: .63rem; color: var(--muted); letter-spacing: .04em; text-transform: uppercase; font-weight: 850; }
.summary-controls { display: flex; gap: 10px; flex-wrap: wrap; }
.compact-select { display: grid; gap: 4px; color: var(--muted); font-size: .78rem; font-weight: 780; }
.compact-select select { min-width: 112px; }

.teacher-panel { padding: clamp(18px, 3vw, 30px); }
.settings-grid { display: grid; grid-template-columns: repeat(3, minmax(170px, 1fr)); gap: 14px; margin-top: 18px; }
.settings-grid label > span { font-size: .82rem; color: var(--muted); }
.toggle-grid { display: grid; grid-template-columns: repeat(2, minmax(220px, 1fr)); gap: 8px 18px; margin: 20px 0; }
.switch-row {
  display: flex;
  align-items: center;
  gap: 10px;
  min-height: 46px;
  padding: 8px 10px;
  border-radius: 11px;
  background: var(--surface-2);
  cursor: pointer;
}
.switch-row input { width: 21px; height: 21px; accent-color: var(--primary); }

.workspace { padding: clamp(16px, 3vw, 30px); overflow: hidden; }
.vocabulary-card { margin-top: 14px; padding: 14px; border: 1px solid var(--line); border-radius: var(--radius-sm); background: var(--surface-2); }
.vocabulary-card dl { display: grid; grid-template-columns: repeat(4, 1fr); gap: 12px; margin: 0; }
.vocabulary-card dl div { padding: 10px; }
.vocabulary-card dt { font-weight: 900; }
.vocabulary-card dd { margin: 3px 0 0; color: var(--muted); }
.model-tabs {
  display: grid;
  grid-template-columns: repeat(4, 1fr);
  gap: 8px;
  margin: 20px 0;
  overflow-x: auto;
  scrollbar-width: thin;
}
.model-tab {
  min-width: 170px;
  min-height: 58px;
  border: 2px solid var(--line);
  border-radius: 15px;
  background: var(--surface-2);
  color: var(--muted);
  padding: 10px 12px;
  display: flex;
  align-items: center;
  gap: 9px;
  font-weight: 850;
  text-align: left;
  cursor: pointer;
}
.model-tab[aria-selected="true"] { border-color: var(--primary); background: var(--primary-soft); color: var(--primary-strong); }
.tab-number { flex: 0 0 29px; width: 29px; height: 29px; display: grid; place-items: center; border-radius: 50%; background: var(--surface); border: 1px solid var(--line); }

.workspace-layout { display: grid; grid-template-columns: minmax(0, 1.55fr) minmax(300px, .65fr); gap: 18px; align-items: stretch; }
.model-stage { min-width: 0; min-height: 530px; border: 2px solid var(--line); border-radius: 20px; background: var(--surface-2); padding: clamp(14px, 2.5vw, 28px); overflow: auto; }
.model-panel { min-height: 100%; }
.model-title-row { display: flex; justify-content: space-between; gap: 12px; align-items: flex-start; margin-bottom: 18px; }
.model-title-row p { margin: 4px 0 0; color: var(--muted); }
.model-badge { padding: 6px 10px; border-radius: 999px; background: var(--primary-soft); color: var(--primary-strong); font-size: .78rem; font-weight: 850; white-space: nowrap; }
.instruction-panel { min-width: 0; border: 2px solid var(--line); border-radius: 20px; background: var(--surface); padding: 20px; display: flex; flex-direction: column; }
.step-counter { background: var(--surface-2); border: 1px solid var(--line); border-radius: 999px; padding: 5px 10px; font-size: .78rem; font-weight: 850; white-space: nowrap; }
.step-explanation { margin: 18px 0; padding: 14px; border-left: 5px solid var(--primary); border-radius: 0 12px 12px 0; background: var(--primary-soft); min-height: 88px; }
.step-explanation p:last-child { margin-bottom: 0; }
.practice-area { min-height: 130px; }
.practice-card { display: grid; gap: 12px; }
.practice-question { font-weight: 850; }
.answer-row { display: flex; gap: 9px; align-items: center; flex-wrap: wrap; }
.answer-row input {
  width: min(220px, 100%);
  border: 2px solid var(--line);
  border-radius: 12px;
  background: var(--surface-2);
  color: var(--text);
  padding: 10px 13px;
  font-size: 1.2rem;
  font-weight: 850;
}
.feedback-message { min-height: 30px; margin: 10px 0; font-weight: 850; }
.feedback-message.correct { color: var(--success); }
.feedback-message.incorrect { color: var(--danger); }
.hint-panel { margin: 10px 0; padding: 13px; border: 2px dashed var(--warning); border-radius: 12px; background: var(--warning-soft); color: color-mix(in srgb, var(--warning) 80%, var(--text)); }
.step-controls { display: grid; grid-template-columns: 1fr 1fr; gap: 10px; margin-top: auto; padding-top: 15px; }
.secondary-step-controls { display: flex; gap: 13px; justify-content: center; flex-wrap: wrap; margin-top: 8px; }

.drag-zone { display: grid; gap: 12px; }
.drag-bank { display: flex; flex-wrap: wrap; gap: 9px; }
.drag-chip {
  min-width: 66px;
  min-height: 50px;
  display: grid;
  place-items: center;
  border: 2px solid var(--primary);
  border-radius: 12px;
  background: var(--primary-soft);
  color: var(--primary-strong);
  font-size: 1.15rem;
  font-weight: 900;
  cursor: grab;
  user-select: none;
}
.drag-chip.selected { outline: 4px solid color-mix(in srgb, var(--focus) 60%, transparent); }
.drop-target {
  min-height: 68px;
  border: 3px dashed var(--line);
  border-radius: 14px;
  background: var(--surface-2);
  display: grid;
  place-items: center;
  padding: 10px;
  font-weight: 800;
  text-align: center;
}
.drop-target.drag-over { border-color: var(--primary); background: var(--primary-soft); }

/* Area model */
.area-wrap { min-width: 560px; }
.area-equation { display: flex; align-items: stretch; gap: 0; margin: 28px auto; width: min(100%, 920px); }
.area-divisor {
  flex: 0 0 82px;
  display: grid;
  place-items: center;
  border-right: 4px solid var(--text);
  font-size: 1.3rem;
  font-weight: 900;
}
.area-boxes { flex: 1; display: grid; grid-auto-flow: column; grid-auto-columns: minmax(130px, 1fr); border-top: 4px solid var(--text); border-bottom: 4px solid var(--text); }
.area-cell { position: relative; min-height: 185px; border-right: 3px solid var(--text); display: grid; place-items: center; padding: 28px 12px 12px; transition: background var(--motion), transform var(--motion); }
.area-cell.highlight { background: var(--primary-soft); transform: translateY(-4px); }
.area-cell .partial-label { position: absolute; top: -38px; left: 0; right: 0; text-align: center; font-size: 1.1rem; font-weight: 900; }
.area-cell .product { font-size: 1.55rem; font-weight: 900; }
.area-cell .product-caption { color: var(--muted); font-size: .78rem; }
.area-total { text-align: center; font-size: 1.15rem; font-weight: 850; }
.remainder-strip { width: min(100%, 920px); margin: 0 auto; padding: 12px; border: 2px dashed var(--warning); border-radius: 12px; background: var(--warning-soft); text-align: center; }

/* Partial quotients */
.partial-table { width: 100%; border-collapse: separate; border-spacing: 0 7px; font-variant-numeric: tabular-nums; }
.partial-table th { color: var(--muted); font-size: .75rem; letter-spacing: .05em; text-transform: uppercase; text-align: right; padding: 0 10px; }
.partial-table th:first-child, .partial-table td:first-child { text-align: left; }
.partial-table td { background: var(--surface); border-top: 1px solid var(--line); border-bottom: 1px solid var(--line); padding: 11px 10px; text-align: right; font-weight: 800; }
.partial-table td:first-child { border-left: 1px solid var(--line); border-radius: 11px 0 0 11px; }
.partial-table td:last-child { border-right: 1px solid var(--line); border-radius: 0 11px 11px 0; }
.partial-table tr.highlight td { background: var(--primary-soft); border-color: var(--primary); }
.partial-sum { margin-top: 15px; padding: 14px; border-top: 3px solid var(--text); text-align: right; font-size: 1.2rem; font-weight: 900; }

/* Standard algorithm */
.long-division-wrap { min-width: 480px; padding: 18px 5px; display: grid; justify-content: center; }
.long-division {
  display: grid;
  grid-template-columns: auto auto;
  grid-template-areas: ". quotient" "divisor dividend" ". work";
  column-gap: 8px;
  width: max-content;
  max-width: 100%;
  font-family: ui-monospace, "SFMono-Regular", Menlo, Consolas, monospace;
  font-size: clamp(1.35rem, 4vw, 2.3rem);
  font-weight: 850;
  font-variant-numeric: tabular-nums;
}
.long-divisor { grid-area: divisor; align-self: start; padding-top: 8px; }
.long-dividend { grid-area: dividend; border-top: 4px solid var(--text); border-left: 4px solid var(--text); border-radius: 16px 0 0 0; padding: 6px 9px 0 12px; letter-spacing: .18em; white-space: pre; }
.long-quotient { grid-area: quotient; padding: 0 9px 4px 12px; letter-spacing: .18em; min-height: 1.5em; white-space: pre; }
.long-work { grid-area: work; padding-left: 12px; letter-spacing: .18em; }
.long-row { min-height: 1.45em; white-space: pre; border-bottom: 2px solid transparent; }
.long-row.subtract { border-bottom-color: var(--text); }
.long-row.highlight { background: var(--primary-soft); border-radius: 8px; }
.algorithm-cycle { display: flex; flex-wrap: wrap; justify-content: center; gap: 8px; margin-top: 24px; }
.algorithm-word { padding: 7px 10px; border: 2px solid var(--line); border-radius: 999px; color: var(--muted); font-size: .76rem; font-weight: 900; text-transform: uppercase; }
.algorithm-word.active { border-color: var(--primary); background: var(--primary-soft); color: var(--primary-strong); }

/* Base-ten blocks */
.blocks-warning { padding: 18px; border: 2px dashed var(--warning); border-radius: 14px; background: var(--warning-soft); }
.place-value-board { display: grid; grid-template-columns: repeat(var(--places, 4), minmax(110px, 1fr)); gap: 10px; min-width: 560px; }
.place-column { border: 2px solid var(--line); border-radius: 14px; background: var(--surface); min-height: 255px; padding: 10px; }
.place-column.highlight { border-color: var(--primary); background: var(--primary-soft); }
.place-heading { text-align: center; font-size: .78rem; font-weight: 900; text-transform: uppercase; color: var(--muted); padding-bottom: 8px; border-bottom: 1px solid var(--line); }
.block-grid { display: flex; flex-wrap: wrap; align-content: flex-start; justify-content: center; gap: 7px; padding-top: 12px; }
.base-block { display: grid; place-items: center; border: 2px solid var(--text); background: var(--surface-2); font-weight: 900; transition: transform var(--motion), opacity var(--motion); }
.base-block.thousand { width: 58px; height: 58px; box-shadow: inset 0 0 0 7px var(--line); }
.base-block.hundred { width: 51px; height: 51px; background-image: linear-gradient(var(--line) 1px, transparent 1px), linear-gradient(90deg, var(--line) 1px, transparent 1px); background-size: 10px 10px; }
.base-block.ten { width: 18px; height: 58px; background-image: repeating-linear-gradient(to bottom, transparent 0 4px, var(--line) 4px 5px); }
.base-block.one { width: 22px; height: 22px; }
.base-block.tenth { width: 18px; height: 50px; opacity: .78; }
.base-block.hundredth { width: 17px; height: 17px; opacity: .72; }
.exchange-note { margin-top: 14px; padding: 11px; border-radius: 11px; background: var(--primary-soft); font-weight: 800; }
.group-preview { margin-top: 16px; display: grid; grid-template-columns: repeat(auto-fit, minmax(105px, 1fr)); gap: 9px; }
.equal-group { min-height: 74px; border: 2px solid var(--line); border-radius: 12px; padding: 9px; text-align: center; background: var(--surface); }
.equal-group strong { display: block; }

.decimal-demo { margin-top: 20px; padding: clamp(16px, 3vw, 26px); border: 2px solid var(--primary); border-radius: 18px; background: var(--primary-soft); }
.shift-badge { background: var(--surface); border: 1px solid var(--line); border-radius: 999px; padding: 6px 10px; font-size: .8rem; font-weight: 900; }
.decimal-animation { display: grid; grid-template-columns: 1fr auto 1fr; align-items: center; gap: 18px; margin: 20px 0; }
.decimal-number-card { background: var(--surface); border: 2px solid var(--line); border-radius: 14px; padding: 14px; text-align: center; overflow: hidden; }
.decimal-number-card .label { color: var(--muted); font-size: .78rem; font-weight: 850; text-transform: uppercase; }
.decimal-number-flow { display: flex; justify-content: center; align-items: center; gap: 10px; margin-top: 8px; font-family: var(--display-font); font-size: clamp(1.25rem, 4vw, 2.1rem); font-weight: 900; }
.move-arrow { font-size: 1.7rem; color: var(--primary); font-weight: 900; animation: decimal-nudge 900ms ease-out both; }
.shifted-value { display: inline-block; animation: decimal-arrive 900ms ease-out both; }
@keyframes decimal-nudge { 0% { transform: translateX(-8px); opacity: .35; } 55% { transform: translateX(5px); opacity: 1; } 100% { transform: translateX(0); opacity: 1; } }
@keyframes decimal-arrive { 0%, 35% { transform: translateX(-12px); opacity: .2; } 100% { transform: translateX(0); opacity: 1; } }

.completion-card { margin-top: 20px; padding: 20px; border: 2px solid var(--success); border-radius: 18px; background: var(--success-soft); display: grid; grid-template-columns: auto 1fr auto; align-items: center; gap: 16px; }
.completion-icon { width: 52px; height: 52px; display: grid; place-items: center; border-radius: 50%; background: var(--success); color: #fff; font-size: 1.6rem; font-weight: 900; }
.completion-card p { margin-bottom: 0; }

.confetti-layer { position: fixed; inset: 0; pointer-events: none; overflow: hidden; z-index: 1000; }
.confetti-piece { position: absolute; width: 10px; height: 16px; border-radius: 2px; animation: confetti-fall 1.7s ease-in forwards; }
@keyframes confetti-fall {
  0% { transform: translate3d(0, -10vh, 0) rotate(0); opacity: 1; }
  100% { transform: translate3d(var(--drift), 110vh, 0) rotate(720deg); opacity: .25; }
}
.pulse-highlight { animation: pulse-highlight 900ms ease-out; }
@keyframes pulse-highlight { 0%,100% { box-shadow: none; } 50% { box-shadow: 0 0 0 7px color-mix(in srgb, var(--primary) 24%, transparent); } }

/* Themes */
body[data-theme="whiteboard"] {
  --bg: #dce8e3; --surface: #fbfffd; --surface-2: #f3faf6; --text: #17342d; --muted: #526a63;
  --line: #c5d6d0; --primary: #126e58; --primary-strong: #0d5544; --primary-soft: #dff3ec;
  --display-font: "Comic Sans MS", "Segoe Print", ui-rounded, sans-serif;
  background-image: radial-gradient(circle at 15% 8%, rgba(255,255,255,.7), transparent 30%);
}
body[data-theme="blackboard"] {
  --bg: #132723; --surface: #1d3933; --surface-2: #17312b; --text: #f4f1dc; --muted: #c6cbbd;
  --line: #4f6f67; --primary: #f1d479; --primary-strong: #ffe699; --primary-soft: #314b3e;
  --success: #9be2b4; --success-soft: #254b39; --danger: #ffb0b0; --danger-soft: #523333; --warning: #f4d37a; --warning-soft: #4d4327;
  --display-font: "Chalkboard SE", "Comic Sans MS", ui-rounded, sans-serif;
  background-image: linear-gradient(104deg, rgba(255,255,255,.015), transparent 55%), radial-gradient(circle at 70% 15%, rgba(255,255,255,.025), transparent 40%);
}
body[data-theme="notebook"] {
  --bg: #ece9df; --surface: #fffdf6; --surface-2: #fbf8ee; --text: #2d3138; --muted: #666b73;
  --line: #cdd8e5; --primary: #2e63aa; --primary-strong: #214b84; --primary-soft: #e7f0fb;
  --display-font: "Segoe Print", "Comic Sans MS", ui-rounded, sans-serif;
  background-image: repeating-linear-gradient(to bottom, transparent 0 31px, rgba(90,140,190,.13) 31px 32px);
}
body[data-theme="notebook"] .problem-card,
body[data-theme="notebook"] .workspace,
body[data-theme="notebook"] .teacher-panel { background-image: linear-gradient(90deg, transparent 0 36px, rgba(207,78,78,.12) 36px 38px, transparent 38px); }

@media (max-width: 1000px) {
  .workspace-layout { grid-template-columns: 1fr; }
  .model-stage { min-height: 460px; }
  .instruction-panel { min-height: 390px; }
  .settings-grid { grid-template-columns: repeat(2, minmax(170px, 1fr)); }
  .vocabulary-card dl { grid-template-columns: repeat(2, 1fr); }
}

@media (max-width: 760px) {
  .app-header { align-items: flex-start; flex-direction: column; padding: 22px 17px 14px; }
  .header-actions { width: 100%; justify-content: stretch; }
  .header-actions .button { flex: 1; }
  .problem-form { grid-template-columns: 1fr auto 1fr; }
  .start-button { grid-column: 1 / -1; }
  .problem-summary-row, .workspace-topbar, .section-heading-row { align-items: flex-start; flex-direction: column; }
  .summary-controls { width: 100%; }
  .compact-select { flex: 1; }
  .settings-grid, .toggle-grid { grid-template-columns: 1fr; }
  .model-tabs { grid-template-columns: repeat(4, minmax(155px, 1fr)); margin-left: -4px; margin-right: -4px; padding: 4px; }
  .model-stage { padding: 12px; }
  .completion-card { grid-template-columns: auto 1fr; }
  .completion-card .button { grid-column: 1 / -1; }
  .decimal-animation { grid-template-columns: 1fr; }
  .decimal-animation > .move-arrow { transform: rotate(90deg); text-align: center; }
}

@media (max-width: 520px) {
  body { font-size: 16px; }
  .app-shell { width: min(100% - 14px, 1480px); gap: 10px; }
  .problem-card, .workspace, .teacher-panel { border-radius: 17px; }
  .problem-form { grid-template-columns: 1fr; }
  .operator { padding: 0; justify-self: center; line-height: .7; }
  .problem-form label, .start-button { grid-column: 1; }
  .equation-display { width: 100%; justify-content: center; }
  .model-stage { min-height: 420px; }
  .instruction-panel { padding: 15px; }
  .step-controls { grid-template-columns: 1fr; }
  .vocabulary-card dl { grid-template-columns: 1fr; }
  .area-wrap, .place-value-board, .long-division-wrap { min-width: 500px; }
}

@media (prefers-reduced-motion: reduce) {
  *, *::before, *::after { animation-duration: .001ms !important; animation-iteration-count: 1 !important; scroll-behavior: auto !important; transition-duration: .001ms !important; }
}
body.reduce-motion *, body.reduce-motion *::before, body.reduce-motion *::after { animation-duration: .001ms !important; animation-iteration-count: 1 !important; transition-duration: .001ms !important; }
body.no-animations * { transition: none !important; animation: none !important; }