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/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>> {