Add service worker implementation and tests for PWA caching strategy

- Implemented a service worker (sw.js) that handles installation, activation, and fetch events.
- Added caching logic for public assets and a fallback for offline navigation.
- Created unit tests (sw.test.ts) to validate service worker behavior, including asset caching, navigation handling, and activation logic.
- Ensured that private API routes and third-party requests are excluded from caching.

src/index.ts

@@ -1,4 +1,5 @@
 import { browserFont } from "./browser-fonts";
+import { pwaAsset } from "./pwa/assets";
 import { startReminders } from "./reminders/worker";
 import { backupConfig, startBackups } from "./ops/backups";
 import { databasePath } from "./db/config";
@@ -30,6 +31,9 @@ const server = Bun.serve({
   hostname: process.env.HOST ?? "127.0.0.1",
   port: Number(process.env.PORT ?? 3000),
   routes: {
+    "/manifest.webmanifest": pwaAsset,
+    "/sw.js": pwaAsset,
+    "/pwa/*": pwaAsset,
     "/fonts/browser/*": browserFont,
     "/api": req => app.fetch(req),
     "/api/*": req => app.fetch(req),