feat: web push notifications for new chapters

- Service worker (src/service-worker.ts) handles push events and
  notification clicks, navigating to the book page on tap
- Web app manifest (manifest.webmanifest) linked in app.html
- Profile page: push notification toggle (subscribe/unsubscribe)
  using the browser Notification + PushManager API with VAPID
- API route POST/DELETE /api/push-subscription proxies to backend
- Go backend: push_subscriptions PocketBase collection storage
  methods (SavePushSubscription, DeletePushSubscription,
  ListPushSubscriptionsByBook) in storage/store.go
- handlers_push.go: GET vapid-public-key, POST/DELETE subscription
- webpush package: VAPID-signed sends via webpush-go, SendToBook
  fans out to all users who have the book in their library
- Runner fires push to subscribers whenever ChaptersScraped > 0
  after a successful book scrape
- Config: VAPID_PUBLIC_KEY, VAPID_PRIVATE_KEY, VAPID_SUBJECT env vars
- domain.ScrapeResult gets a Slug field; orchestrator populates it

ui/src/service-worker.ts

@@ -0,0 +1,73 @@
+/// <reference types="@sveltejs/kit" />
+/// <reference lib="webworker" />
+
+declare let self: ServiceWorkerGlobalScope;
+
+// ── Install / Activate ────────────────────────────────────────────────────────
+
+self.addEventListener('install', () => {
+	self.skipWaiting();
+});
+
+self.addEventListener('activate', (event) => {
+	event.waitUntil(self.clients.claim());
+});
+
+// ── Push notifications ────────────────────────────────────────────────────────
+
+interface PushPayload {
+	title: string;
+	body: string;
+	url?: string;
+	icon?: string;
+	badge?: string;
+}
+
+self.addEventListener('push', (event) => {
+	if (!event.data) return;
+
+	let payload: PushPayload;
+	try {
+		payload = event.data.json() as PushPayload;
+	} catch {
+		payload = { title: 'LibNovel', body: event.data.text() };
+	}
+
+	const options: NotificationOptions = {
+		body: payload.body,
+		icon: payload.icon ?? '/icon-192.png',
+		badge: payload.badge ?? '/favicon-32.png',
+		data: { url: payload.url ?? '/' },
+		// Show notification even when the app is focused
+		requireInteraction: false,
+	};
+
+	event.waitUntil(
+		self.registration.showNotification(payload.title, options)
+	);
+});
+
+// ── Notification click ────────────────────────────────────────────────────────
+
+self.addEventListener('notificationclick', (event) => {
+	event.notification.close();
+
+	const url: string = (event.notification.data as { url?: string })?.url ?? '/';
+
+	event.waitUntil(
+		self.clients
+			.matchAll({ type: 'window', includeUncontrolled: true })
+			.then((clientList) => {
+				// Focus existing window if it has the target URL already open
+				for (const client of clientList) {
+					if (client.url === url && 'focus' in client) {
+						return client.focus();
+					}
+				}
+				// Otherwise open a new window
+				if (self.clients.openWindow) {
+					return self.clients.openWindow(url);
+				}
+			})
+	);
+});