Word Search: extract pure selection/match logic + pin with vitest

Per Codex's Phase 2 audit notes. Moved the drag-snap (lineFrom) and find-match
(matchWord) logic into $lib/wordsearch.js and added vitest coverage:
- lineFrom always yields a straight, in-bounds path — a non-straight drag snaps,
  never returns bent; single cell and edge-clamping covered.
- matchWord matches forward + reversed selections, is a harmless no-op on an
  already-found word (so completion/best-time can't double-record), and returns
  null for non-words / too-short selections.

Restore behaviour audited: finish() (which records best-time) only runs when the
final word is found mid-play; on refresh, restore() repopulates found cells +
time and the derived status flips to done WITHOUT calling finish(), so best-time
never re-records. First JS test runner for the frontend (npm test → vitest run).

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

frontend/package.json

@@ -6,7 +6,8 @@
   "scripts": {
     "dev": "vite dev --host",
     "build": "vite build",
-    "preview": "vite preview"
+    "preview": "vite preview",
+    "test": "vitest run"
   },
   "devDependencies": {
     "@sveltejs/adapter-static": "^3.0.6",
@@ -14,6 +15,7 @@
     "@sveltejs/vite-plugin-svelte": "^5.0.0",
     "an-array-of-english-words": "^2.0.0",
     "svelte": "^5.1.0",
-    "vite": "^6.0.0"
+    "vite": "^6.0.0",
+    "vitest": "^4.1.8"
   }
 }