feat(sw): implement and enable robust service worker caching

- Re-enabled automatic service worker registration in svelte.config.js.
- Implemented a production-ready service-worker.js using SvelteKit metadata (-worker).
- Added intelligent caching for build artifacts, static files, and network fallbacks.
- This ensures the UI shell and assets load instantly from disk, completing the hydration performance workstream.

svelte.config.js

@@ -15,7 +15,7 @@ const config = {
     },
     kit: {
         serviceWorker: {
-            register: false
+            register: true
         },
         adapter: adapter_node()