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/e2e/game.spec.ts

@@ -48,6 +48,18 @@ test('a pending tile recalls on double-tap, not on a single tap', async ({ page
   await expect(page.locator('[data-cell].pending')).toHaveCount(0);
 });
 
+test('the board is a gapless checkerboard by default; grid lines toggle in Settings', async ({ page }) => {
+  await openGame(page);
+  await expect(page.locator('.grid.gridless')).toBeVisible(); // lines off by default
+
+  await page.evaluate(() => (location.hash = '/settings'));
+  await page.locator('.gridlines input').check(); // turn grid lines on
+  await page.evaluate(() => (location.hash = '/game/g1'));
+
+  await expect(page.locator('.grid')).toBeVisible();
+  await expect(page.locator('.grid.gridless')).toHaveCount(0);
+});
+
 test('shuffle reorders the rack but keeps the same tiles', async ({ page }) => {
   await openGame(page);
   const before = await page.locator('.rack .tile').allTextContents();