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.

src/service-worker.js

@@ -1,13 +1,64 @@
 /// <reference types="@sveltejs/kit" />
-/// <reference lib="webworker" />
+import { build, files, version } from '$service-worker';
 
-console.log('Service Worker: Hello from the simplified JS worker!');
+// Create a unique cache name for this deployment
+const CACHE = `cache-${version}`;
 
-self.addEventListener('install', () => {
-    console.log('Service Worker: Installed');
-    self.skipWaiting();
+const ASSETS = [
+	...build, // the app itself
+	...files  // everything in `static`
+];
+
+self.addEventListener('install', (event) => {
+	// Create a new cache and add all files to it
+	async function addFilesToCache() {
+		const cache = await caches.open(CACHE);
+		await cache.addAll(ASSETS);
+	}
+
+	event.waitUntil(addFilesToCache());
 });
 
-self.addEventListener('activate', () => {
-    console.log('Service Worker: Activated');
+self.addEventListener('activate', (event) => {
+	// Remove previous cached data from other screens
+	async function deleteOldCaches() {
+		for (const key of await caches.keys()) {
+			if (key !== CACHE) await caches.delete(key);
+		}
+	}
+
+	event.waitUntil(deleteOldCaches());
 });
+
+self.addEventListener('fetch', (event) => {
+	// ignore POST requests etc
+	if (event.request.method !== 'GET') return;
+
+	async function respond() {
+		const url = new URL(event.request.url);
+		const cache = await caches.open(CACHE);
+
+		// `build`/`files` can always be served from the cache
+		if (ASSETS.includes(url.pathname)) {
+			const cachedResponse = await cache.match(url.pathname);
+            if (cachedResponse) return cachedResponse;
+		}
+
+		// for everything else, try the network first, but fall back to the cache if we're offline
+		try {
+			const response = await fetch(event.request);
+
+			if (response.status === 200) {
+				cache.put(event.request, response.clone());
+			}
+
+			return response;
+		} catch (err) {
+			const cachedResponse = await cache.match(event.request);
+            if (cachedResponse) return cachedResponse;
+            throw err;
+		}
+	}
+
+	event.respondWith(respond());
+});

svelte.config.js

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