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>

next.config.ts

@@ -1,7 +1,30 @@
 import type { NextConfig } from "next";
+import withPWAInit from "@ducanh2912/next-pwa";
+
+const withPWA = withPWAInit({
+  dest: "public",
+  cacheOnFrontEndNav: true,
+  aggressiveFrontEndNavCaching: true,
+  reloadOnOnline: true,
+  disable: process.env.NODE_ENV === "development",
+  workboxOptions: {
+    disableDevLogs: true,
+    runtimeCaching: [
+      {
+        urlPattern: /^https?.*/,
+        handler: "NetworkFirst",
+        options: {
+          cacheName: "grow-api-cache",
+          expiration: { maxEntries: 200, maxAgeSeconds: 24 * 60 * 60 },
+          networkTimeoutSeconds: 10,
+        },
+      },
+    ],
+  },
+});
 
 const nextConfig: NextConfig = {
   output: "standalone",
 };
 
-export default nextConfig;
+export default withPWA(nextConfig);