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/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',