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/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>> {