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>

src/app/layout.tsx

@@ -10,6 +10,10 @@ export const metadata: Metadata = {
     capable: true,
     statusBarStyle: "default",
     title: "Grow",
+    startupImage: "/apple-touch-icon.png",
+  },
+  icons: {
+    apple: "/apple-touch-icon.png",
   },
 };