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/screens/Settings.svelte

@@ -3,6 +3,7 @@
   import {
     app,
     setBoardLabels,
+    setBoardLines,
     setLocalePref,
     setReduceMotion,
     setTheme,
@@ -63,6 +64,14 @@
           </button>
         {/each}
       </div>
+      <label class="row gridlines">
+        <span>{t('settings.boardLines')}</span>
+        <input
+          type="checkbox"
+          checked={app.boardLines}
+          onchange={(e) => setBoardLines(e.currentTarget.checked)}
+        />
+      </label>
     </section>
 
     <section>
@@ -118,4 +127,7 @@
     align-items: center;
     justify-content: space-between;
   }
+  .gridlines {
+    margin-top: 12px;
+  }
 </style>