test: add frontend test suite with 54 tests

- Install vitest, @testing-library/svelte, jsdom
- Configure vitest in vite.config.js with browser conditions for Svelte 5
- Pure function tests for api.js (artworkUrl, bundleUrl, setupBundleUrl)
- Router tests (push, replace hash manipulation)
- GameCard component tests (title, year, genre, cover, badge, click)
- Library page tests (loading, empty, game grid, year/genre filters)
- GameDetail page tests (loading, error, display, edit mode, badges)
- Play page tests (loading, not-ready, unsupported, booting, error)

frontend/package.json

@@ -6,11 +6,16 @@
   "scripts": {
     "dev": "vite",
     "build": "vite build",
-    "preview": "vite preview"
+    "preview": "vite preview",
+    "test": "vitest run",
+    "test:watch": "vitest"
   },
   "devDependencies": {
     "@sveltejs/vite-plugin-svelte": "^4.0.0",
+    "@testing-library/svelte": "^5.3.1",
+    "jsdom": "^29.1.1",
     "svelte": "^5.0.0",
-    "vite": "^5.0.0"
+    "vite": "^5.0.0",
+    "vitest": "^4.1.8"
   }
 }