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/api.ts

@@ -18,6 +18,8 @@ export function createApi(db: AppDatabase, config: AuthConfig, request?: FetchDi
   app.use("*", async (c, next) => {
     const requestId = crypto.randomUUID();
     c.set("requestId", requestId); c.header("X-Request-ID", requestId);
+    // Accounts, calendars, exports and OAuth responses must never enter HTTP caches.
+    c.header("Cache-Control", "no-store");
     await next();
   });
   const auth = createAuth(db, config, request, now);