feat(pwa): full PWA setup with service worker and PNG icons

- Add @ducanh2912/next-pwa with NetworkFirst runtime caching (24h, 200 entries)
- Disabled in development to avoid SW interference
- Generate icon-192.png, icon-512.png, apple-touch-icon.png from SVG
- Update manifest.json to use PNG icons (required for Android install prompt)
- Add apple-touch-icon + appleWebApp metadata to layout
- Add /offline fallback page shown when network unavailable

Offline behavior: cached reads work on all platforms; background sync
for new events not supported (iOS Safari has no Background Sync API).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

public/manifest.json

@@ -9,9 +9,15 @@
   "orientation": "portrait",
   "icons": [
     {
-      "src": "/icon.svg",
-      "sizes": "any",
-      "type": "image/svg+xml",
+      "src": "/icon-192.png",
+      "sizes": "192x192",
+      "type": "image/png",
+      "purpose": "any"
+    },
+    {
+      "src": "/icon-512.png",
+      "sizes": "512x512",
+      "type": "image/png",
       "purpose": "any maskable"
     }
   ]