/* ============================================================
   TOKENS & RESET
   ============================================================ */
:root {
  --c-bg:          #f0f4ff;
  --c-surface:     #ffffff;
  --c-border:      #d1d9e6;
  --c-primary:     #4361ee;
  --c-primary-dim: #eef0fd;
  --c-accent:      #4cc9f0;
  --c-error:       #ef233c;
  --c-error-dim:   #fff0f2;
  --c-text:        #1a1d2e;
  --c-muted:       #8892a4;
  --c-given:       #1a1d2e;
  --c-user:        #4361ee;
  --c-note:        #4895ef;
  --c-highlight:   #eef0fd;
  --c-selected:    #c7cffe;
  --c-same:        #dde1fd;
  --c-box-border:  #7b8ab8;

  --cell:          56px;
  --board:         calc(var(--cell) * 9);
  --radius:        12px;
  --shadow:        0 4px 24px rgba(67, 97, 238, 0.12);
}

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

body {
  font-family: 'Segoe UI', system-ui, -apple-system, sans-serif;
  background: var(--c-bg);
  color: var(--c-text);
  min-height: 100dvh;
  display: flex;
  align-items: flex-start;
  justify-content: center;
}

/* ============================================================
   APP WRAPPER
   ============================================================ */
.app {
  width: 100%;
  max-width: 900px;
  padding: 16px;
  display: flex;
  flex-direction: column;
  gap: 14px;
}

/* ============================================================
   HEADER
   ============================================================ */
.header {
  display: flex;
  align-items: center;
  justify-content: space-between;
  background: var(--c-surface);
  border-radius: var(--radius);
  padding: 14px 20px;
  box-shadow: var(--shadow);
}

.logo {
  font-size: 1.5rem;
  font-weight: 800;
  color: var(--c-primary);
  letter-spacing: -0.5px;
}

.header-right {
  display: flex;
  gap: 16px;
}

.stat-box {
  display: flex;
  flex-direction: column;
  align-items: center;
  gap: 2px;
}

.stat-label {
  font-size: 0.65rem;
  font-weight: 600;
  text-transform: uppercase;
  letter-spacing: 0.5px;
  color: var(--c-muted);
}

.stat-value {
  font-size: 1.1rem;
  font-weight: 700;
  font-variant-numeric: tabular-nums;
  color: var(--c-text);
}

.mistake-dots {
  display: flex;
  gap: 5px;
  margin-top: 2px;
}

.mistake-dot {
  width: 10px;
  height: 10px;
  border-radius: 50%;
  background: var(--c-border);
  transition: background 0.3s, transform 0.2s;
}

.mistake-dot.active {
  background: var(--c-error);
  transform: scale(1.15);
}

/* ============================================================
   DIFFICULTY BAR
   ============================================================ */
.difficulty-bar {
  display: flex;
  gap: 8px;
  background: var(--c-surface);
  border-radius: var(--radius);
  padding: 8px;
  box-shadow: var(--shadow);
}

.diff-btn {
  flex: 1;
  padding: 8px 4px;
  border: none;
  border-radius: 8px;
  background: transparent;
  color: var(--c-muted);
  font-size: 0.85rem;
  font-weight: 600;
  cursor: pointer;
  transition: background 0.2s, color 0.2s;
}

.diff-btn:hover { background: var(--c-highlight); color: var(--c-primary); }
.diff-btn.active {
  background: var(--c-primary);
  color: #fff;
}

/* ============================================================
   MAIN LAYOUT
   ============================================================ */
.main {
  display: flex;
  gap: 16px;
  align-items: flex-start;
}

/* ============================================================
   SUDOKU BOARD
   ============================================================ */
.sudoku-board {
  display: grid;
  grid-template-columns: repeat(9, var(--cell));
  grid-template-rows:    repeat(9, var(--cell));
  background: var(--c-surface);
  border-radius: var(--radius);
  box-shadow: var(--shadow);
  overflow: hidden;
  flex-shrink: 0;
  border: 2px solid var(--c-box-border);
}

.cell {
  width:  var(--cell);
  height: var(--cell);
  display: flex;
  align-items: center;
  justify-content: center;
  font-size: 1.25rem;
  font-weight: 600;
  cursor: pointer;
  user-select: none;
  position: relative;
  color: var(--c-user);
  transition: background 0.1s;
  border-right:  1px solid var(--c-border);
  border-bottom: 1px solid var(--c-border);
}

/* Remove last column / last row borders */
.cell:nth-child(9n)   { border-right:  none; }
.cell:nth-last-child(-n+9) { border-bottom: none; }

/* Thick box borders */
.cell[data-col="3"],
.cell[data-col="6"] { border-left: 2px solid var(--c-box-border); }

.cell[data-row="3"],
.cell[data-row="6"] { border-top:  2px solid var(--c-box-border); }

/* States */
.cell.given      { color: var(--c-given); font-weight: 700; }
.cell.highlighted { background: var(--c-highlight); }
.cell.selected   { background: var(--c-selected) !important; }
.cell.same-number { background: var(--c-same); }

.cell.error      { color: var(--c-error); }
.cell.error-bg   { background: var(--c-error-dim); }

/* Notes grid */
.note-grid {
  display: grid;
  grid-template-columns: repeat(3, 1fr);
  grid-template-rows:    repeat(3, 1fr);
  width: 100%;
  height: 100%;
  padding: 2px;
  gap: 0;
}

.note-cell {
  display: flex;
  align-items: center;
  justify-content: center;
  font-size: calc(var(--cell) * 0.19);
  font-weight: 600;
  color: var(--c-note);
  line-height: 1;
}

/* Hint pulse */
.cell.hint-flash {
  animation: hintFlash 0.5s ease;
}

@keyframes hintFlash {
  0%,100% { background: var(--c-selected); }
  50%      { background: #b5f7c4; }
}

/* ============================================================
   SIDEBAR
   ============================================================ */
.sidebar {
  flex: 1;
  display: flex;
  flex-direction: column;
  gap: 12px;
}

/* Action buttons */
.action-bar {
  display: grid;
  grid-template-columns: 1fr 1fr;
  gap: 8px;
}

.action-btn {
  display: flex;
  flex-direction: column;
  align-items: center;
  gap: 4px;
  padding: 10px 4px;
  border: 1.5px solid var(--c-border);
  border-radius: 10px;
  background: var(--c-surface);
  color: var(--c-muted);
  font-size: 0.72rem;
  font-weight: 600;
  cursor: pointer;
  transition: border-color 0.2s, color 0.2s, background 0.2s;
}

.action-btn:hover {
  border-color: var(--c-primary);
  color: var(--c-primary);
  background: var(--c-primary-dim);
}

.action-btn.active {
  border-color: var(--c-primary);
  color: var(--c-primary);
  background: var(--c-primary-dim);
}

.action-btn:disabled {
  opacity: 0.4;
  cursor: not-allowed;
}

/* Number pad */
.number-pad {
  display: grid;
  grid-template-columns: repeat(3, 1fr);
  gap: 8px;
}

.num-btn {
  aspect-ratio: 1;
  display: flex;
  flex-direction: column;
  align-items: center;
  justify-content: center;
  gap: 2px;
  border: 1.5px solid var(--c-border);
  border-radius: 10px;
  background: var(--c-surface);
  cursor: pointer;
  transition: border-color 0.15s, background 0.15s, transform 0.1s;
}

.num-btn:hover:not(.completed) {
  border-color: var(--c-primary);
  background: var(--c-primary-dim);
  transform: translateY(-1px);
}

.num-btn:active { transform: scale(0.95); }

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

.num-label {
  font-size: 1.5rem;
  font-weight: 700;
  color: var(--c-primary);
}

.num-count {
  font-size: 0.6rem;
  font-weight: 600;
  color: var(--c-muted);
  min-height: 10px;
}

/* ============================================================
   DIALOGS
   ============================================================ */
.dialog-overlay {
  position: fixed;
  inset: 0;
  background: rgba(26, 29, 46, 0.5);
  display: flex;
  align-items: center;
  justify-content: center;
  z-index: 100;
  opacity: 0;
  pointer-events: none;
  backdrop-filter: blur(4px);
  transition: opacity 0.25s;
}

.dialog-overlay.show {
  opacity: 1;
  pointer-events: all;
}

.dialog {
  background: var(--c-surface);
  border-radius: 20px;
  padding: 36px 32px;
  text-align: center;
  max-width: 360px;
  width: 90%;
  transform: scale(0.9) translateY(20px);
  transition: transform 0.25s;
  box-shadow: 0 20px 60px rgba(0,0,0,0.25);
}

.dialog-overlay.show .dialog {
  transform: scale(1) translateY(0);
}

.dialog-icon {
  font-size: 3.5rem;
  margin-bottom: 12px;
}

.dialog-title {
  font-size: 1.8rem;
  font-weight: 800;
  margin-bottom: 6px;
}

.dialog-subtitle {
  color: var(--c-muted);
  margin-bottom: 24px;
}

.dialog-stats {
  display: flex;
  justify-content: space-around;
  background: var(--c-bg);
  border-radius: 12px;
  padding: 16px;
  margin-bottom: 24px;
}

.dialog-stat {
  display: flex;
  flex-direction: column;
  gap: 4px;
}

.dialog-stat-label {
  font-size: 0.7rem;
  font-weight: 600;
  text-transform: uppercase;
  letter-spacing: 0.5px;
  color: var(--c-muted);
}

.dialog-stat-value {
  font-size: 1.2rem;
  font-weight: 700;
  font-variant-numeric: tabular-nums;
}

.dialog-actions {
  display: flex;
  flex-direction: column;
  gap: 10px;
}

.dialog-btn {
  width: 100%;
  padding: 13px;
  border: none;
  border-radius: 10px;
  font-size: 0.95rem;
  font-weight: 700;
  cursor: pointer;
  transition: opacity 0.15s, transform 0.1s;
}

.dialog-btn:hover { opacity: 0.85; }
.dialog-btn:active { transform: scale(0.98); }

.dialog-btn.primary {
  background: var(--c-primary);
  color: #fff;
}

.dialog-btn.secondary {
  background: var(--c-bg);
  color: var(--c-text);
}

/* ============================================================
   LOADING
   ============================================================ */
.loading-overlay {
  position: fixed;
  inset: 0;
  background: rgba(240, 244, 255, 0.85);
  display: flex;
  flex-direction: column;
  align-items: center;
  justify-content: center;
  gap: 16px;
  z-index: 200;
  opacity: 0;
  pointer-events: none;
  transition: opacity 0.2s;
  font-weight: 600;
  color: var(--c-muted);
  backdrop-filter: blur(2px);
}

.loading-overlay.show {
  opacity: 1;
  pointer-events: all;
}

.loading-spinner {
  width: 40px;
  height: 40px;
  border: 3px solid var(--c-border);
  border-top-color: var(--c-primary);
  border-radius: 50%;
  animation: spin 0.8s linear infinite;
}

@keyframes spin { to { transform: rotate(360deg); } }

/* ============================================================
   RESPONSIVE — tablet / mobile
   ============================================================ */
@media (max-width: 700px) {
  :root { --cell: 38px; }

  .main { flex-direction: column; align-items: center; }

  .sidebar { width: 100%; max-width: calc(var(--cell) * 9); }

  .action-bar { grid-template-columns: repeat(4, 1fr); }

  .number-pad { grid-template-columns: repeat(9, 1fr); gap: 4px; }

  .num-btn { aspect-ratio: auto; padding: 8px 4px; }

  .num-count { display: none; }
}

@media (max-width: 420px) {
  :root { --cell: 36px; }
  .logo { font-size: 1.2rem; }
  .difficulty-bar { gap: 4px; }
  .diff-btn { font-size: 0.75rem; padding: 6px 2px; }
}