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.

README.md

@@ -1,5 +1,9 @@
 # Mina Habits
 
+The production app is installable as a PWA, with an offline reconnect screen and
+user-controlled updates. Habit data and changes require a connection. See
+[PWA behavior and verification](docs/PWA.md).
+
 Bun + Hono habit-tracking REST API with Drizzle ORM and local SQLite. The React Router
 client includes a homepage for signed-out visitors and a personal habit dashboard.