* { margin: 0; padding: 0; box-sizing: border-box; }

:root, [data-theme="midnight"] {
  --bg: #0a0a10;
  --bg2: #0e0e16;
  --surface: #151520;
  --surface-hover: #1c1c2e;
  --border: #252538;
  --border-box: #4a3f8a;
  --accent: #6c5ce7;
  --accent-glow: #a29bfe;
  --accent-dim: rgba(108, 92, 231, 0.15);
  --text: #e0e0e8;
  --text-dim: #55556a;
  --given: #f0f0f5;
  --input: #a29bfe;
  --error: #ff6b6b;
  --success: #51cf66;
  --radius: 8px;
  --grad1: rgba(108, 92, 231, 0.04);
  --grad2: rgba(162, 155, 254, 0.03);
  --selected-bg: rgba(108, 92, 231, 0.30);
  --highlight-bg: rgba(108, 92, 231, 0.12);
  --same-bg: rgba(162, 155, 254, 0.22);
  --error-bg: rgba(255, 107, 107, 0.12);
  --glow1: rgba(108, 92, 231, 0.3);
  --glow2: rgba(162, 155, 254, 0.5);
  --board-shadow: rgba(108, 92, 231, 0.06);
}

[data-theme="nord"] {
  --bg: #2e3440;
  --bg2: #3b4252;
  --surface: #434c5e;
  --surface-hover: #4c566a;
  --border: #4c566a;
  --border-box: #5e81ac;
  --accent: #88c0d0;
  --accent-glow: #8fbcbb;
  --accent-dim: rgba(136, 192, 208, 0.15);
  --text: #eceff4;
  --text-dim: #7b88a1;
  --given: #eceff4;
  --input: #88c0d0;
  --error: #bf616a;
  --success: #a3be8c;
  --grad1: rgba(136, 192, 208, 0.05);
  --grad2: rgba(143, 188, 187, 0.04);
  --selected-bg: rgba(136, 192, 208, 0.30);
  --highlight-bg: rgba(136, 192, 208, 0.14);
  --same-bg: rgba(143, 188, 187, 0.24);
  --error-bg: rgba(191, 97, 106, 0.15);
  --glow1: rgba(136, 192, 208, 0.3);
  --glow2: rgba(143, 188, 187, 0.5);
  --board-shadow: rgba(136, 192, 208, 0.06);
}

[data-theme="rose"] {
  --bg: #191724;
  --bg2: #1f1d2e;
  --surface: #26233a;
  --surface-hover: #2a2740;
  --border: #393552;
  --border-box: #6e6a86;
  --accent: #eb6f92;
  --accent-glow: #ebbcba;
  --accent-dim: rgba(235, 111, 146, 0.15);
  --text: #e0def4;
  --text-dim: #6e6a86;
  --given: #e0def4;
  --input: #ebbcba;
  --error: #eb6f92;
  --success: #9ccfd8;
  --grad1: rgba(235, 111, 146, 0.04);
  --grad2: rgba(235, 188, 186, 0.03);
  --selected-bg: rgba(235, 111, 146, 0.28);
  --highlight-bg: rgba(235, 111, 146, 0.12);
  --same-bg: rgba(235, 188, 186, 0.22);
  --error-bg: rgba(235, 111, 146, 0.15);
  --glow1: rgba(235, 111, 146, 0.3);
  --glow2: rgba(235, 188, 186, 0.5);
  --board-shadow: rgba(235, 111, 146, 0.06);
}

[data-theme="paper"] {
  --bg: #f5f0e8;
  --bg2: #ebe6dc;
  --surface: #e0dbd1;
  --surface-hover: #d6d0c5;
  --border: #c8c2b6;
  --border-box: #8a8478;
  --accent: #7c6f5b;
  --accent-glow: #5c5347;
  --accent-dim: rgba(124, 111, 91, 0.15);
  --text: #3a3530;
  --text-dim: #9a9488;
  --given: #2a2520;
  --input: #7c6f5b;
  --error: #c44040;
  --success: #5a8a5a;
  --grad1: rgba(124, 111, 91, 0.06);
  --grad2: rgba(92, 83, 71, 0.04);
  --selected-bg: rgba(124, 111, 91, 0.28);
  --highlight-bg: rgba(124, 111, 91, 0.13);
  --same-bg: rgba(124, 111, 91, 0.20);
  --error-bg: rgba(196, 64, 64, 0.12);
  --glow1: rgba(124, 111, 91, 0.2);
  --glow2: rgba(92, 83, 71, 0.3);
  --board-shadow: rgba(0, 0, 0, 0.05);
}

[data-theme="cyberpunk"] {
  --bg: #090712;
  --bg2: #130d22;
  --surface: #19122d;
  --surface-hover: #22183b;
  --border: #34285b;
  --border-box: #00d9ff;
  --accent: #ff3d81;
  --accent-glow: #00f0ff;
  --accent-dim: rgba(255, 61, 129, 0.14);
  --text: #f4f1ff;
  --text-dim: #72689e;
  --given: #fcfaff;
  --input: #00f0ff;
  --error: #ff6f91;
  --success: #72f1b8;
  --grad1: rgba(255, 61, 129, 0.08);
  --grad2: rgba(0, 240, 255, 0.06);
  --selected-bg: rgba(255, 61, 129, 0.26);
  --highlight-bg: rgba(0, 240, 255, 0.12);
  --same-bg: rgba(255, 61, 129, 0.16);
  --error-bg: rgba(255, 111, 145, 0.15);
  --glow1: rgba(255, 61, 129, 0.38);
  --glow2: rgba(0, 240, 255, 0.45);
  --board-shadow: rgba(0, 240, 255, 0.12);
}

[data-theme="mono"] {
  --bg: #111215;
  --bg2: #17191d;
  --surface: #1e2025;
  --surface-hover: #272a30;
  --border: #353942;
  --border-box: #5a606b;
  --accent: #c7cad1;
  --accent-glow: #f3f4f6;
  --accent-dim: rgba(199, 202, 209, 0.14);
  --text: #eceef2;
  --text-dim: #7e838d;
  --given: #ffffff;
  --input: #dadde4;
  --error: #8f949d;
  --success: #f2f4f7;
  --grad1: rgba(255, 255, 255, 0.04);
  --grad2: rgba(199, 202, 209, 0.05);
  --selected-bg: rgba(243, 244, 246, 0.18);
  --highlight-bg: rgba(199, 202, 209, 0.10);
  --same-bg: rgba(218, 221, 228, 0.18);
  --error-bg: rgba(143, 148, 157, 0.14);
  --glow1: rgba(243, 244, 246, 0.16);
  --glow2: rgba(255, 255, 255, 0.22);
  --board-shadow: rgba(255, 255, 255, 0.05);
}

html, body {
  height: 100%;
  overflow: hidden;
}

html {
  transition: background-color 0.3s, color 0.3s;
}

body {
  font-family: 'SF Mono', 'Fira Code', 'Cascadia Code', 'Consolas', monospace;
  background-color: var(--bg);
  background-image:
    radial-gradient(ellipse at 20% 50%, var(--grad1) 0%, transparent 60%),
    radial-gradient(ellipse at 80% 50%, var(--grad2) 0%, transparent 60%);
  color: var(--text);
  display: flex;
  justify-content: center;
  align-items: center;
}

/* ── Welcome screen ──────────────────────────────────────────── */

#welcome {
  position: fixed;
  inset: 0;
  display: flex;
  align-items: center;
  justify-content: center;
  z-index: 200;
  background-color: var(--bg);
  background-image:
    radial-gradient(ellipse at 30% 40%, var(--grad1) 0%, transparent 50%),
    radial-gradient(ellipse at 70% 60%, var(--grad2) 0%, transparent 50%);
  transition: opacity 0.4s ease, transform 0.4s ease;
}

#welcome[hidden] { display: none; }

#welcome.fade-out {
  opacity: 0;
  transform: scale(1.02);
  pointer-events: none;
}

#welcome-inner {
  display: flex;
  flex-direction: column;
  align-items: center;
  gap: 2rem;
  padding: 2rem;
  max-width: 360px;
  width: 100%;
}

#welcome-brand {
  display: flex;
  flex-direction: column;
  align-items: center;
  gap: 0.6rem;
}

#welcome-logo {
  width: 80px;
  height: 80px;
  display: flex;
  align-items: center;
  justify-content: center;
  font-size: 2.4rem;
  font-weight: 200;
  color: var(--accent-glow);
  border: 2px solid var(--border-box);
  border-radius: 16px;
  background: var(--bg2);
  box-shadow: 0 0 40px var(--board-shadow), inset 0 0 20px var(--board-shadow);
  animation: logo-breathe 4s ease-in-out infinite;
}

@keyframes logo-breathe {
  0%, 100% { box-shadow: 0 0 40px var(--board-shadow), inset 0 0 20px var(--board-shadow); }
  50% { box-shadow: 0 0 60px var(--glow1), inset 0 0 30px var(--board-shadow); }
}

#welcome-title {
  font-size: 1.6rem;
  letter-spacing: 0.4em;
  color: var(--accent-glow);
  text-transform: uppercase;
  font-weight: 300;
}

#welcome-title .dim { color: var(--text-dim); }

#welcome-sub {
  font-size: 0.7rem;
  color: var(--text-dim);
  letter-spacing: 0.15em;
  text-transform: uppercase;
}

#welcome-actions {
  display: flex;
  flex-direction: column;
  gap: 0.5rem;
  width: 100%;
}

.w-btn {
  width: 100%;
  padding: 0.7rem;
  border-radius: var(--radius);
  font-family: inherit;
  font-size: 0.85rem;
  letter-spacing: 0.08em;
  cursor: pointer;
  transition: all 0.2s;
  text-align: center;
  border: 1px solid var(--border);
}

.w-btn[hidden] { display: none; }

.w-primary {
  background: var(--accent);
  color: #fff;
  border-color: var(--accent);
}

.w-primary:hover {
  box-shadow: 0 0 20px var(--glow1);
  transform: translateY(-1px);
}

.w-secondary {
  background: var(--surface);
  color: var(--text);
}

.w-secondary:hover {
  border-color: var(--accent);
  background: var(--surface-hover);
}

#welcome-diff {
  display: flex;
  gap: 0.4rem;
}

.w-diff {
  padding: 0.35rem 1.2rem;
  border-radius: 20px;
  background: var(--surface);
  color: var(--text-dim);
  border: 1px solid var(--border);
  font-family: inherit;
  font-size: 0.65rem;
  letter-spacing: 0.08em;
  text-transform: uppercase;
  cursor: pointer;
  transition: all 0.2s;
}

.w-diff:hover { border-color: var(--accent); color: var(--text); }

.w-diff.active {
  background: var(--accent);
  color: #fff;
  border-color: var(--accent);
}

#welcome-footer {
  display: flex;
  flex-direction: column;
  align-items: center;
  gap: 0.8rem;
  width: 100%;
}

#welcome-themes {
  display: flex;
  gap: 0.5rem;
  flex-wrap: wrap;
  justify-content: center;
}

#welcome-stats {
  font-size: 0.6rem;
  color: var(--text-dim);
  letter-spacing: 0.06em;
  text-align: center;
}

/* ── Layout ─────────────────────────────────────────────────── */

#app {
  display: flex;
  flex-direction: column;
  height: 100dvh;
  max-height: 100dvh;
  width: 100%;
  max-width: 780px;
  padding: 1rem 1.2rem;
  gap: 0.6rem;
}

#topbar {
  display: flex;
  justify-content: space-between;
  align-items: center;
  flex-shrink: 0;
}

#topbar-right {
  display: flex;
  align-items: center;
  gap: 1rem;
}

#theme-picker {
  display: flex;
  gap: 0.4rem;
  align-items: center;
  flex-wrap: wrap;
  justify-content: flex-end;
}

.theme-dot {
  width: 20px;
  height: 20px;
  border-radius: 50%;
  border: 2px solid var(--border);
  background: transparent;
  cursor: pointer;
  padding: 0;
  display: flex;
  align-items: center;
  justify-content: center;
  transition: border-color 0.2s, transform 0.15s;
}

.theme-dot span {
  width: 10px;
  height: 10px;
  border-radius: 50%;
  display: block;
}

.theme-dot:hover {
  border-color: var(--accent);
  transform: scale(1.15);
}

.theme-dot.active {
  border-color: var(--accent-glow);
  box-shadow: 0 0 8px var(--glow1);
}

h1 {
  font-size: 1.2rem;
  letter-spacing: 0.35em;
  color: var(--accent-glow);
  text-transform: uppercase;
  font-weight: 300;
}

h1 .dim { color: var(--text-dim); }

#timer-wrap {
  display: flex;
  align-items: center;
  gap: 0.4rem;
}

#timer {
  font-size: 0.95rem;
  color: var(--text-dim);
  letter-spacing: 0.12em;
  font-variant-numeric: tabular-nums;
}

#btn-pause {
  background: none;
  border: none;
  color: var(--text-dim);
  cursor: pointer;
  font-size: 0.85rem;
  padding: 0.15rem;
  transition: color 0.15s;
  line-height: 1;
}

#btn-pause:hover { color: var(--accent); }
#btn-pause.paused { color: var(--accent); }

#game {
  display: flex;
  gap: 1.2rem;
  flex: 1;
  min-height: 0;
  align-items: center;
}

/* ── Board ──────────────────────────────────────────────────── */

#board-wrap {
  flex-shrink: 0;
  display: flex;
  align-items: center;
  justify-content: center;
  position: relative;
}

#pause-overlay {
  position: absolute;
  inset: 0;
  background: var(--bg);
  display: flex;
  flex-direction: column;
  align-items: center;
  justify-content: center;
  gap: 1rem;
  border-radius: 6px;
  z-index: 10;
  opacity: 0.97;
}

#pause-overlay[hidden] { display: none; }

#pause-overlay span {
  font-size: 1.4rem;
  letter-spacing: 0.4em;
  color: var(--text-dim);
  font-weight: 300;
}

#btn-resume {
  background: var(--surface);
  color: var(--text);
  border: 1px solid var(--border);
  padding: 0.5rem 1.6rem;
  border-radius: var(--radius);
  font-family: inherit;
  font-size: 0.8rem;
  cursor: pointer;
  letter-spacing: 0.06em;
  transition: all 0.2s;
}

#btn-resume:hover { border-color: var(--accent); background: var(--surface-hover); }

#board {
  --cell: min(calc((100dvh - 8rem) / 9.6), 56px);
  display: grid;
  grid-template-columns: repeat(9, var(--cell));
  grid-template-rows: repeat(9, var(--cell));
  border: 2px solid var(--border-box);
  border-radius: 6px;
  overflow: hidden;
  box-shadow: 0 0 40px var(--board-shadow);
}

.cell {
  display: flex;
  align-items: center;
  justify-content: center;
  font-size: calc(var(--cell) * 0.44);
  border: 1px solid var(--border);
  cursor: pointer;
  transition: background 0.12s, color 0.12s;
  user-select: none;
  background: var(--bg2);
}

/* 3x3 box borders */
.cell:nth-child(3n)          { border-right: 2px solid var(--border-box); }
.cell:nth-child(9n)          { border-right: none; }
.cell:nth-child(n+19):nth-child(-n+27),
.cell:nth-child(n+46):nth-child(-n+54) { border-bottom: 2px solid var(--border-box); }

.cell.given { color: var(--given); font-weight: 600; }
.cell.input { color: var(--input); }

.cell.selected {
  background: var(--selected-bg);
  box-shadow: inset 0 0 0 2px var(--accent);
}

.cell.highlight { background: var(--highlight-bg); }
.cell.same-num { background: var(--same-bg); }

.cell.error {
  color: var(--error);
  background: var(--error-bg);
  animation: shake 0.3s;
}

/* ── Sidebar ────────────────────────────────────────────────── */

#sidebar {
  display: flex;
  flex-direction: column;
  gap: 0.7rem;
  flex: 1;
  min-width: 0;
  justify-content: center;
  max-width: 200px;
}

#score-bar {
  display: flex;
  justify-content: space-between;
  align-items: center;
  padding: 0.4rem 0.6rem;
  background: var(--surface);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  font-size: 0.7rem;
  letter-spacing: 0.06em;
}

#score-label { color: var(--text-dim); text-transform: uppercase; }
#score-value { color: var(--accent-glow); font-variant-numeric: tabular-nums; }

#controls {
  display: flex;
  gap: 0.35rem;
}

.diff-btn {
  flex: 1;
  background: var(--surface);
  color: var(--text-dim);
  border: 1px solid var(--border);
  padding: 0.45rem 0;
  border-radius: var(--radius);
  cursor: pointer;
  font-family: inherit;
  font-size: 0.65rem;
  letter-spacing: 0.08em;
  text-transform: uppercase;
  transition: all 0.2s;
  text-align: center;
}

.diff-btn:hover { border-color: var(--accent); color: var(--text); }
.diff-btn.active {
  background: var(--accent);
  color: #fff;
  border-color: var(--accent);
  box-shadow: 0 0 12px var(--glow1);
}

#numpad {
  display: grid;
  grid-template-columns: repeat(3, 1fr);
  gap: 0.35rem;
}

.num-btn {
  aspect-ratio: 1;
  background: var(--surface);
  color: var(--text);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  font-family: inherit;
  font-size: 1.15rem;
  cursor: pointer;
  transition: all 0.15s;
  position: relative;
  display: flex;
  align-items: center;
  justify-content: center;
}

.num-btn:hover {
  border-color: var(--accent);
  background: var(--surface-hover);
}

.num-btn.active-num {
  background: var(--accent-dim);
  border-color: var(--accent);
  color: var(--accent-glow);
  box-shadow: 0 0 10px var(--glow1);
}

.num-btn.completed {
  opacity: 0.3;
  pointer-events: none;
}

.num-btn .count {
  position: absolute;
  top: 3px;
  right: 4px;
  font-size: 0.5rem;
  color: var(--text-dim);
  line-height: 1;
}

.num-btn.erase {
  grid-column: 1 / -1;
  aspect-ratio: auto;
  padding: 0.5rem;
  color: var(--error);
  font-size: 0.85rem;
}

#actions {
  display: grid;
  grid-template-columns: repeat(4, 1fr);
  gap: 0.35rem;
}

#actions button {
  background: var(--surface);
  color: var(--text);
  border: 1px solid var(--border);
  padding: 0.5rem 0;
  border-radius: var(--radius);
  font-family: inherit;
  font-size: 0.7rem;
  cursor: pointer;
  letter-spacing: 0.04em;
  transition: all 0.2s;
  text-align: center;
}

#actions button:hover { border-color: var(--accent); background: var(--surface-hover); }

#btn-hint { position: relative; }
#hint-count {
  font-size: 0.55rem;
  color: var(--accent-glow);
  margin-left: 0.15rem;
}
#btn-hint.exhausted { opacity: 0.35; pointer-events: none; }

.cell.candidates {
  font-size: calc(var(--cell) * 0.2);
  line-height: 1.15;
  color: var(--text-dim);
  white-space: pre-wrap;
  text-align: center;
  letter-spacing: 0.05em;
}

.cell.hint-reveal {
  color: var(--success);
  animation: reveal-pop 0.4s ease-out;
}

/* ── Learn challenge system ──────────────────────────────────── */

#learn-overlay {
  position: fixed;
  inset: 0;
  background: rgba(0,0,0,0.7);
  display: flex;
  align-items: center;
  justify-content: center;
  z-index: 100;
  backdrop-filter: blur(4px);
}
#learn-overlay[hidden] { display: none; }

#learn-panel {
  background: var(--bg);
  border: 1px solid var(--border);
  border-radius: 12px;
  padding: 1.2rem;
  max-width: 480px;
  width: 90vw;
  max-height: 85dvh;
  overflow-y: auto;
  scrollbar-width: thin;
  scrollbar-color: var(--border) transparent;
}

#learn-panel::-webkit-scrollbar { width: 6px; }
#learn-panel::-webkit-scrollbar-track { background: transparent; }
#learn-panel::-webkit-scrollbar-thumb { background: var(--border); border-radius: 3px; }

#learn-header {
  display: flex;
  align-items: center;
  gap: 0.6rem;
  margin-bottom: 1rem;
}
#learn-header h2 {
  font-size: 1rem;
  font-weight: 400;
  letter-spacing: 0.15em;
  color: var(--success);
  flex: 1;
}
#learn-stars-total { font-size: 0.7rem; color: var(--text-dim); }
#learn-close {
  background: none; border: none; color: var(--text-dim);
  font-size: 1rem; cursor: pointer; padding: 0.2rem;
}
#learn-close:hover { color: var(--text); }

.learn-stage-label {
  font-size: 0.6rem; color: var(--text-dim); text-transform: uppercase;
  letter-spacing: 0.1em; margin: 0.8rem 0 0.4rem;
}
.learn-stage-label:first-child { margin-top: 0; }

.learn-grid { display: grid; grid-template-columns: repeat(4, 1fr); gap: 0.35rem; }

.challenge-card {
  background: var(--surface); border: 1px solid var(--border);
  border-radius: var(--radius); padding: 0.5rem 0.3rem;
  cursor: pointer; text-align: center; transition: all 0.2s;
}
.challenge-card:hover { border-color: var(--accent); background: var(--surface-hover); }
.challenge-card.locked { opacity: 0.3; pointer-events: none; }
.challenge-card.completed { border-color: var(--success); }
.challenge-card .ch-num { font-size: 0.85rem; color: var(--text); font-weight: 600; }
.challenge-card .ch-title {
  font-size: 0.45rem; color: var(--text-dim); margin-top: 0.1rem;
  text-transform: uppercase; letter-spacing: 0.03em;
}
.challenge-card .ch-stars { font-size: 0.55rem; margin-top: 0.15rem; }

#learn-lesson { padding: 0.5rem 0; }
#learn-lesson[hidden] { display: none; }
#lesson-badge {
  font-size: 0.6rem; color: var(--accent-glow); text-transform: uppercase;
  letter-spacing: 0.1em; margin-bottom: 0.3rem;
}
#lesson-title { font-size: 1rem; font-weight: 400; color: var(--text); margin-bottom: 0.5rem; }
#lesson-desc { font-size: 0.75rem; color: var(--text-dim); line-height: 1.5; margin-bottom: 0.5rem; }
.lesson-grid {
  display: grid;
  grid-template-columns: repeat(2, minmax(0, 1fr));
  gap: 0.45rem;
  margin-bottom: 0.75rem;
}
.lesson-block {
  background: var(--surface);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  padding: 0.55rem 0.6rem;
}
.lesson-label {
  display: block;
  font-size: 0.55rem;
  color: var(--accent-glow);
  text-transform: uppercase;
  letter-spacing: 0.08em;
  margin-bottom: 0.25rem;
}
.lesson-block p {
  font-size: 0.68rem;
  color: var(--text-dim);
  line-height: 1.45;
}
.tip {
  font-size: 0.7rem; color: var(--success); padding: 0.5rem 0.6rem;
  background: var(--surface); border-left: 3px solid var(--success);
  border-radius: 0 var(--radius) var(--radius) 0; margin-bottom: 0.8rem;
}
#lesson-actions {
  display: flex;
  gap: 0.35rem;
}
#lesson-back { flex: 1; }
#lesson-start { flex: 2; }

/* Unified learn button — mirrors #actions button */
.learn-btn {
  background: var(--surface);
  color: var(--text);
  border: 1px solid var(--border);
  padding: 0.5rem 0;
  border-radius: var(--radius);
  font-family: inherit;
  font-size: 0.7rem;
  cursor: pointer;
  letter-spacing: 0.04em;
  transition: all 0.2s;
  text-align: center;
}
.learn-btn:hover { border-color: var(--accent); background: var(--surface-hover); }
.learn-btn-primary {
  background: var(--accent);
  color: #fff;
  border-color: var(--accent);
}
.learn-btn-primary:hover {
  box-shadow: 0 0 12px var(--glow1);
  background: var(--accent);
}
.learn-btn[hidden] { display: none; }

/* Learn completion view */
#learn-complete { text-align: center; padding: 1.5rem 0; }
#learn-complete[hidden] { display: none; }
#complete-stars { font-size: 2rem; margin-bottom: 0.5rem; }
#complete-title { font-size: 1rem; font-weight: 400; color: var(--text); margin-bottom: 0.3rem; }
#complete-msg { font-size: 0.7rem; color: var(--text-dim); margin-bottom: 1rem; line-height: 1.4; }
#complete-actions { display: flex; gap: 0.35rem; }
#complete-back { flex: 1; }
#complete-next { flex: 2; }

/* Challenge done bar */
#challenge-done {
  display: none;
  justify-content: center;
  gap: 0.35rem;
  flex-shrink: 0;
  animation: pop 0.2s ease-out;
}
#challenge-done:not([hidden]) { display: flex; }
#done-next { flex: 2; }
#done-list { flex: 1; }

/* 4×4 board override */
#board.board-4 {
  --cell: min(calc((100dvh - 12rem) / 5), 72px);
  grid-template-columns: repeat(4, var(--cell));
  grid-template-rows: repeat(4, var(--cell));
}
#board.board-4 .cell:nth-child(2n) { border-right: 2px solid var(--border-box); }
#board.board-4 .cell:nth-child(4n) { border-right: none; }
#board.board-4 .cell:nth-child(3n) { border-right: 1px solid var(--border); }
#board.board-4 .cell:nth-child(n+5):nth-child(-n+8) { border-bottom: 2px solid var(--border-box); }

/* When in 4×4 challenge, switch #game to centered column layout */
#game.game-mini {
  flex-direction: column;
  align-items: center;
  justify-content: center;
  gap: 0.8rem;
}

#game.game-mini #board-wrap {
  flex-shrink: 0;
}

#game.game-mini #sidebar {
  max-width: 320px;
  width: 100%;
  align-items: center;
}

#game.game-mini #numpad {
  grid-template-columns: repeat(4, 1fr);
  max-width: 220px;
}

#game.game-mini .num-btn.erase {
  grid-column: 1 / -1;
}

/* Challenge info bar */
#challenge-bar {
  display: none;
  justify-content: space-between;
  align-items: center;
  gap: 0.35rem;
  padding: 0.4rem 0.6rem;
  background: var(--surface);
  border: 1px solid var(--success);
  border-radius: var(--radius);
  font-size: 0.65rem;
  letter-spacing: 0.04em;
}
#challenge-bar.visible { display: flex; }
#challenge-bar .ch-info { color: var(--success); flex: 1; min-width: 0; }
#challenge-bar .ch-tip-btn {
  background: rgba(255,255,255,0.02);
  border: 1px solid var(--border);
  border-radius: 999px;
  color: var(--text-dim);
  cursor: pointer;
  font-size: 0.62rem;
  font-family: inherit;
  padding: 0.18rem 0.5rem;
  white-space: nowrap;
}
#challenge-bar .ch-tip-btn:hover { color: var(--accent); }

.cell.pencil-marks {
  font-size: calc(var(--cell) * 0.18);
  line-height: 1.1;
  color: var(--text-dim);
  white-space: pre-wrap;
  text-align: center;
  letter-spacing: 0;
  padding: 1px;
  word-spacing: 0.1em;
}

.cell.guide-cell {
  animation: guide-pulse 1.5s ease-in-out infinite;
}

@keyframes guide-pulse {
  0%, 100% { box-shadow: inset 0 0 0 2px transparent; }
  50% { box-shadow: inset 0 0 0 2px var(--success); }
}

.cell.correct-pop {
  animation: correct-ripple 0.5s ease-out;
}

@keyframes correct-ripple {
  0% { box-shadow: inset 0 0 0 0 var(--success); }
  40% { box-shadow: inset 0 0 12px 2px var(--success); }
  100% { box-shadow: none; }
}

.cell.wrong-flash {
  animation: wrong-flash 0.5s ease-out;
  color: var(--error);
}

@keyframes wrong-flash {
  0% { background: var(--error-bg); }
  100% { background: var(--bg2); }
}

#streak-bar {
  display: none;
  justify-content: space-between;
  align-items: center;
  padding: 0.4rem 0.6rem;
  background: var(--surface);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  font-size: 0.7rem;
  letter-spacing: 0.06em;
}

#streak-bar.visible { display: flex; }
#streak-label { color: var(--text-dim); text-transform: uppercase; }
#streak-value { color: var(--success); font-variant-numeric: tabular-nums; }

.diff-btn[data-diff="learn"] {
  border-color: var(--success);
  color: var(--success);
}
.diff-btn[data-diff="learn"]:hover {
  background: var(--surface-hover);
}
.diff-btn[data-diff="learn"].active {
  background: var(--success);
  color: var(--bg);
  border-color: var(--success);
  box-shadow: 0 0 12px rgba(81, 207, 102, 0.3);
}

@keyframes reveal-pop {
  0% { transform: scale(1.4); opacity: 0.5; }
  100% { transform: scale(1); opacity: 1; }
}

#best-scores {
  display: flex;
  justify-content: space-between;
  font-size: 0.55rem;
  color: var(--text-dim);
  letter-spacing: 0.04em;
  text-transform: uppercase;
  padding: 0 0.2rem;
}

#best-scores .best { font-variant-numeric: tabular-nums; }

/* ── Message ────────────────────────────────────────────────── */

#message {
  font-size: 0.8rem;
  min-height: 1.2em;
  color: var(--success);
  letter-spacing: 0.04em;
  text-align: center;
  flex-shrink: 0;
}

#message.error { color: var(--error); }

#confirm-bar {
  display: none;
  align-items: center;
  justify-content: center;
  gap: 0.5rem;
  flex-shrink: 0;
  animation: pop 0.15s ease-out;
}
#confirm-bar:not([hidden]) { display: flex; }
#confirm-text {
  font-size: 0.75rem;
  color: var(--text);
  letter-spacing: 0.03em;
}
#confirm-yes, #confirm-no {
  background: var(--surface);
  color: var(--text);
  border: 1px solid var(--border);
  padding: 0.25rem 0.7rem;
  border-radius: var(--radius);
  font-family: inherit;
  font-size: 0.65rem;
  cursor: pointer;
  transition: all 0.15s;
}
#confirm-yes {
  background: var(--error);
  color: #fff;
  border-color: var(--error);
}
#confirm-yes:hover { opacity: 0.85; }
#confirm-no:hover { border-color: var(--accent); }

/* ── Animations ─────────────────────────────────────────────── */

@keyframes shake {
  0%, 100% { transform: translateX(0); }
  25% { transform: translateX(-3px); }
  75% { transform: translateX(3px); }
}

@keyframes pop {
  0% { transform: scale(0.8); opacity: 0; }
  100% { transform: scale(1); opacity: 1; }
}

.cell.pop { animation: pop 0.15s ease-out; }

@keyframes glow {
  0%, 100% { box-shadow: 0 0 12px var(--glow1); }
  50% { box-shadow: 0 0 36px var(--glow2); }
}

#board.won { animation: glow 1.5s ease-in-out 3; }

/* ── Focus ──────────────────────────────────────────────────── */

.cell:focus-visible,
.num-btn:focus-visible,
.diff-btn:focus-visible,
#actions button:focus-visible,
.learn-btn:focus-visible,
.theme-dot:focus-visible {
  outline: 2px solid var(--accent-glow);
  outline-offset: -2px;
}

/* ── Mobile: vertical compact layout ────────────────────────── */

@media (max-width: 640px) {
  /* Welcome */
  #welcome-inner { padding: 1.2rem; gap: 1.5rem; }
  #welcome-logo { width: 64px; height: 64px; font-size: 2rem; }
  #welcome-title { font-size: 1.3rem; }
  .w-diff { padding: 0.3rem 0.9rem; }

  /* App layout */
  #app {
    padding: 0.6rem 0.5rem;
    gap: 0.4rem;
  }

  h1 { font-size: 0.9rem; letter-spacing: 0.25em; }

  #topbar-right { gap: 0.5rem; }
  #theme-picker { gap: 0.25rem; }
  .theme-dot { width: 16px; height: 16px; }
  .theme-dot span { width: 8px; height: 8px; }

  #game {
    flex-direction: column;
    gap: 0.5rem;
  }

  #board {
    --cell: min(calc((100vw - 1.5rem) / 9), calc((100dvh - 14rem) / 9));
  }

  #board.board-4 {
    --cell: min(calc((100vw - 2rem) / 4.5), 64px);
  }

  #sidebar {
    max-width: 100%;
    width: 100%;
    gap: 0.4rem;
  }

  #numpad {
    grid-template-columns: repeat(5, 1fr);
    gap: 0.3rem;
  }

  .num-btn {
    aspect-ratio: 1.3;
    font-size: 1rem;
  }

  .num-btn.erase {
    grid-column: span 1;
    aspect-ratio: 1.3;
  }

  #controls { gap: 0.25rem; }
  .diff-btn { padding: 0.3rem 0; font-size: 0.55rem; }

  #actions { gap: 0.25rem; grid-template-columns: repeat(4, 1fr); }
  #actions button { padding: 0.35rem 0; font-size: 0.6rem; }

  #score-bar, #streak-bar, #challenge-bar { padding: 0.3rem 0.5rem; font-size: 0.6rem; }
  #best-scores { font-size: 0.5rem; }
  #message { font-size: 0.7rem; }
  #confirm-text { font-size: 0.65rem; }
  #confirm-yes, #confirm-no { font-size: 0.6rem; padding: 0.2rem 0.5rem; }

  /* Learn overlay mobile */
  #learn-overlay { align-items: flex-start; padding-top: env(safe-area-inset-top, 0); }

  #learn-panel {
    width: 100vw;
    max-width: 100vw;
    max-height: 100dvh;
    border-radius: 0;
    padding: 0.8rem;
    border: none;
  }

  #learn-header { margin-bottom: 0.6rem; }
  #learn-header h2 { font-size: 0.85rem; }
  #learn-stars-total { font-size: 0.6rem; }

  .learn-grid { gap: 0.25rem; grid-template-columns: repeat(3, 1fr); }
  .challenge-card { padding: 0.4rem 0.2rem; }
  .challenge-card .ch-num { font-size: 0.8rem; }
  .challenge-card .ch-title { font-size: 0.42rem; }
  .challenge-card .ch-stars { font-size: 0.5rem; }

  #lesson-desc { font-size: 0.7rem; }
  .lesson-grid { grid-template-columns: 1fr; gap: 0.35rem; }
  .lesson-block { padding: 0.45rem 0.5rem; }
  .lesson-block p { font-size: 0.63rem; }
  .tip { font-size: 0.65rem; padding: 0.4rem 0.5rem; }
  #lesson-actions { gap: 0.25rem; }
  .learn-btn { font-size: 0.65rem; padding: 0.4rem 0; }

  /* Mini game on mobile */
  #game.game-mini #numpad {
    grid-template-columns: repeat(4, 1fr);
    max-width: 200px;
  }

  #game.game-mini .num-btn {
    aspect-ratio: 1;
    font-size: 1rem;
  }

  #game.game-mini .num-btn.erase {
    grid-column: 1 / -1;
    aspect-ratio: auto;
    padding: 0.4rem;
  }

  #challenge-bar { font-size: 0.55rem; }
  #challenge-bar .ch-tip-btn { font-size: 0.52rem; padding: 0.16rem 0.42rem; }
}

/* ── Reduced motion ─────────────────────────────────────────── */

@media (prefers-reduced-motion: reduce) {
  *, *::before, *::after {
    animation-duration: 0.01ms !important;
    animation-iteration-count: 1 !important;
    transition-duration: 0.01ms !important;
  }
}