Stage 17 (#12): lines-off board variant (gapless checkerboard), Settings toggle

Add a 'Grid lines' preference (default off): when off the board drops the 1px grid
gaps for a gapless checkerboard (plain cells alternate shades; tiles get rounded
corners and a soft right-side shadow so adjacent gapless tiles still read apart),
saving ~14px of width. When on, the classic lined grid returns. Persisted with the
other board-style prefs; wired through Board's new lines prop. e2e locks the default
and the toggle.

ui/src/game/Board.svelte

@@ -16,6 +16,7 @@
     zoomed,
     variant,
     labelMode,
+    lines,
     locale,
     focus,
     oncell,
@@ -32,6 +33,8 @@
     zoomed: boolean;
     variant: Variant;
     labelMode: BoardLabelMode;
+    /** Draw 1px grid lines between cells; when false the board is a gapless checkerboard. */
+    lines: boolean;
     locale: Locale;
     focus: { row: number; col: number } | null;
     oncell: (row: number, col: number) => void;
@@ -94,7 +97,7 @@
 
 <div class="viewport" class:zoomed bind:this={viewport}>
   <div class="scaler" style="--z: {z};">
-    <div class="grid">
+    <div class="grid" class:gridless={!lines}>
       {#each board as rowCells, r (r)}
         {#each rowCells as cell, c (c)}
           {@const p = pending.get(key(r, c))}
@@ -107,6 +110,7 @@
             class:pending={!!p && !cell}
             class:hl={!!cell && highlight.has(key(r, c)) && !flash}
             class:flash={!!cell && flash && highlight.has(key(r, c))}
+            class:dark={premium[r][c] === '' && !cell && !p && (r + c) % 2 === 1}
             data-cell
             data-row={r}
             data-col={c}
@@ -187,6 +191,28 @@
   .cell.pending {
     background: var(--tile-pending);
   }
+  /* Lines-off variant: a gapless checkerboard. The 1px grid gaps (and the cell-line they
+     reveal) collapse, saving ~14px of board width; plain cells alternate shades, and tiles
+     get rounded corners and a soft right-side shadow so adjacent gapless tiles still read
+     as separate pieces. */
+  .grid.gridless {
+    gap: 0;
+    padding: 0;
+    background: var(--board-bg);
+  }
+  .grid.gridless .cell {
+    border-radius: 0;
+  }
+  .grid.gridless .cell.dark {
+    background: color-mix(in srgb, var(--cell-bg) 88%, #000);
+  }
+  .grid.gridless .cell.filled,
+  .grid.gridless .cell.pending {
+    border-radius: 4px;
+    box-shadow:
+      inset 0 -2px 0 var(--tile-edge),
+      2px 0 3px -1px rgba(0, 0, 0, 0.4);
+  }
   .cell.hl {
     background: var(--tile-recent);
     /* The bottom edge goes darker than the highlighted fill (not lighter, as the plain

ui/src/game/Game.svelte

@@ -576,6 +576,7 @@
           {zoomed}
           {variant}
           labelMode={app.boardLabels}
+          lines={app.boardLines}
           locale={app.locale}
           {focus}
           oncell={onCell}

ui/src/lib/app.svelte.ts

@@ -40,6 +40,8 @@ export const app = $state<{
   locale: Locale;
   reduceMotion: boolean;
   boardLabels: BoardLabelMode;
+  /** Draw grid lines between board cells; off (default) is a gapless checkerboard. */
+  boardLines: boolean;
   localeLocked: boolean;
   /** Pending incoming friend requests + invitations, for the lobby badge. */
   notifications: number;
@@ -53,6 +55,7 @@ export const app = $state<{
   locale: 'en',
   reduceMotion: false,
   boardLabels: 'beginner',
+  boardLines: false,
   localeLocked: false,
   notifications: 0,
 });
@@ -229,6 +232,7 @@ export async function bootstrap(): Promise<void> {
   app.theme = prefs.theme ?? 'auto';
   app.reduceMotion = prefs.reduceMotion ?? false;
   app.boardLabels = prefs.boardLabels ?? 'beginner';
+  app.boardLines = prefs.boardLines ?? false;
   applyTheme(app.theme);
   applyReduceMotion(app.reduceMotion);
   if (prefs.locale) {
@@ -354,6 +358,7 @@ function persistPrefs(): void {
     locale: app.locale,
     reduceMotion: app.reduceMotion,
     boardLabels: app.boardLabels,
+    boardLines: app.boardLines,
   });
 }
 
@@ -406,6 +411,11 @@ export function setBoardLabels(mode: BoardLabelMode): void {
   persistPrefs();
 }
 
+export function setBoardLines(on: boolean): void {
+  app.boardLines = on;
+  persistPrefs();
+}
+
 // Background/foreground lifecycle: silence the reconnect banner during a suspend and
 // reconnect quietly on return (and refresh the lobby badge for any push missed while
 // hidden, §10). Several signals cover the platforms: the page Visibility API, the

ui/src/lib/i18n/en.ts

@@ -139,6 +139,7 @@ export const en = {
   'settings.labelsBeginner': 'Beginner',
   'settings.labelsClassic': 'Classic',
   'settings.labelsNone': 'None',
+  'settings.boardLines': 'Grid lines',
   'settings.reduceMotion': 'Reduce motion',
 
   'about.title': 'About',

ui/src/lib/i18n/ru.ts

@@ -140,6 +140,7 @@ export const ru: Record<MessageKey, string> = {
   'settings.labelsBeginner': 'Новичок',
   'settings.labelsClassic': 'Классика',
   'settings.labelsNone': 'Без текста',
+  'settings.boardLines': 'Линии сетки',
   'settings.reduceMotion': 'Меньше анимаций',
 
   'about.title': 'О программе',

ui/src/lib/session.ts

@@ -124,6 +124,8 @@ export interface Prefs {
   locale: Locale;
   reduceMotion: boolean;
   boardLabels: BoardLabelMode;
+  /** Draw the 1px grid lines between cells; off (default) shows a gapless checkerboard. */
+  boardLines: boolean;
 }
 
 export async function loadPrefs(): Promise<Partial<Prefs>> {

ui/src/screens/Settings.svelte

@@ -3,6 +3,7 @@
   import {
     app,
     setBoardLabels,
+    setBoardLines,
     setLocalePref,
     setReduceMotion,
     setTheme,
@@ -63,6 +64,14 @@
           </button>
         {/each}
       </div>
+      <label class="row gridlines">
+        <span>{t('settings.boardLines')}</span>
+        <input
+          type="checkbox"
+          checked={app.boardLines}
+          onchange={(e) => setBoardLines(e.currentTarget.checked)}
+        />
+      </label>
     </section>
 
     <section>
@@ -118,4 +127,7 @@
     align-items: center;
     justify-content: space-between;
   }
+  .gridlines {
+    margin-top: 12px;
+  }
 </style>