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/Game.svelte

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