feat: Phase 5.3 - PWA Offline Support with Workbox

- Add vite-plugin-pwa + workbox-window
- Configure PWA manifest with icons (SVG)
- Configure Workbox runtime caching:
  - API routes: NetworkFirst (10s timeout, 100 entries, 24h)
  - Media files: CacheFirst (50 entries, 7 days)
  - Pages: NetworkFirst (10s timeout, 50 entries, 24h)
- Register service worker in app.js with update notification
- Generate icons (SVG) for all sizes

Tests: 101 passing (319 assertions)

package.json

@@ -18,7 +18,9 @@
         "postcss": "^8.5.12",
         "pusher-js": "^8.6.0",
         "tailwindcss": "^3.4.19",
-        "vite": "^7.0.7"
+        "vite": "^7.0.7",
+        "vite-plugin-pwa": "^1.3.0",
+        "workbox-window": "^7.4.1"
     },
     "dependencies": {
         "@tailwindcss/typography": "^0.5.19",