Extract + unit-test the padding-aware cell geometry (Codex nice-to-have)

Pulled the pointer→cell math out of cellAt() into a pure cellFromPoint(rect, x,
y, n) in $lib/wordsearch.js (only getBoundingClientRect stays in the component),
and covered it with vitest — including the last-column case that was drifting
under the old overflowing layout, plus clamping and a scrolled-origin rect.
11 vitest tests now; real-device testing remains the final validator.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/lib/components/WordSearchGame.svelte

@@ -1,6 +1,6 @@
 <script>
   import { getJSON } from '$lib/api.js';
-  import { lineFrom, matchWord } from '$lib/wordsearch.js';
+  import { lineFrom, matchWord, cellFromPoint } from '$lib/wordsearch.js';
 
   let { size = 'med', onstatus } = $props();
 
@@ -86,12 +86,7 @@
   function summary() { return { game: 'wordsearch', size, date, status, found: foundWords.length, total: words.length, ms: resultMs }; }
 
   function cellAt(e) {
-    const rect = gridEl.getBoundingClientRect();
-    const pad = 7;                                  // grid padding (6) + border (1)
-    const cw = (rect.width - 2 * pad) / n;          // even 1fr columns share the inner width
-    const c = Math.min(n - 1, Math.max(0, Math.floor((e.clientX - rect.left - pad) / cw)));
-    const r = Math.min(n - 1, Math.max(0, Math.floor((e.clientY - rect.top - pad) / cw)));
-    return [r, c];
+    return cellFromPoint(gridEl.getBoundingClientRect(), e.clientX, e.clientY, n);
   }
 
   function down(e) {