feat(offline): hold through a short network drop with a caching worker

The service worker did Web Push and nothing else, with a note that a cache
serving yesterday's data was worse than an honest "cannot connect". This adds a
cache that does not do that: network-first for everything that can go stale, so a
reader online always gets the live answer and the cache is reached only when the
network cannot answer. The task and note lists stay live; they just no longer go
blank the moment the signal does — a reload through a tunnel, a lift or a
base-station handover comes back to the app instead of the browser's offline page.

Writes are untouched and still go straight to the network; api.js already retries
them across the same short drop. The shell cache is versioned so a deploy lands in
a fresh cache and the old one is dropped on activate; the data cache is cleared on
sign-out so the next person on the browser does not inherit it.

The old hand-registered /sw.js becomes SvelteKit's src/service-worker.js, which it
registers itself, and the push handlers move across unchanged.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

web/src/lib/components/Sidebar.svelte

@@ -31,6 +31,16 @@
 
 	async function signOut() {
 		await api.logout();
+		// Empty the offline data cache before leaving: the service worker keeps the
+		// last-seen task and note lists so a network drop does not blank the app, and
+		// the next person to sign in on this browser must not find them.
+		try {
+			navigator.serviceWorker?.controller?.postMessage({ type: 'clear-api-cache' });
+			await caches?.delete('verdande-api');
+		} catch {
+			// No service worker / no Cache API (older browser, private mode): nothing
+			// was cached, so nothing to clear.
+		}
 		location.href = '/';
 	}