refactor(notification): provide notification store via context + fixture

Converts the module-singleton notificationStore into a context-provided
store so its specs can drive it without mocking the module. notifications.svelte
now exports createNotificationStore() (the former singleton body), plus
provideNotificationStore()/getNotificationStore()/NOTIFICATION_KEY mirroring
the confirm service. Root +layout provides it; NotificationBell and the
Chronik page read it via getNotificationStore().

Tests:
- notifications.svelte.spec drives a fresh createNotificationStore() per test
  (replacing __resetForTest/__setNavigateForTest with setNavigate()).
- notification.test-fixture.svelte wraps the bell, provides the store, and
  exposes setNotifications(items) via onReady (option b).
- NotificationBell.svelte.spec asserts the announced unread count across the
  empty / single / many / error a11y states (AC#5), stubbing EventSource+fetch.
- aktivitaeten page spec injects a real store via render context.

Per the recorded Phase-2b decision (full context refactor). Part of #560.

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

frontend/src/lib/notification/notifications.svelte.ts

@@ -1,121 +1,161 @@
+import { getContext, setContext } from 'svelte';
 import { type NotificationItem, parseNotificationEvent } from '$lib/notification/notifications';
 
 export type { NotificationItem };
 
-let notifications = $state<NotificationItem[]>([]);
-let unreadCount = $state(0);
-let eventSource: EventSource | null = null;
-let refCount = 0;
-let errorCount = 0;
-let navigate: (url: string) => void = (url) => {
-	window.location.href = url;
-};
+export const NOTIFICATION_KEY = Symbol('notification');
 
-async function fetchNotifications(): Promise<void> {
-	try {
-		const res = await fetch('/api/notifications?size=10');
-		if (res.ok) {
-			const data = await res.json();
-			notifications = data.content ?? [];
-		}
-	} catch (e) {
-		console.error('Failed to fetch notifications', e);
-	}
+export interface NotificationStore {
+	readonly notifications: NotificationItem[];
+	readonly unreadCount: number;
+	fetchNotifications(): Promise<void>;
+	fetchUnreadCount(): Promise<void>;
+	optimisticMarkRead(id: string): void;
+	optimisticMarkAllRead(): void;
+	init(): void;
+	destroy(): void;
+	/** Test-only: seed the notification list and derive the unread count. */
+	setNotifications(items: NotificationItem[]): void;
+	/** Test-only: override the 401 → redirect side-effect. */
+	setNavigate(fn: (url: string) => void): void;
 }
 
-async function fetchUnreadCount(): Promise<void> {
-	try {
-		const res = await fetch('/api/notifications/unread-count');
-		if (res.ok) {
-			const data = await res.json();
-			unreadCount = data.count;
-		}
-	} catch (e) {
-		console.error('Failed to fetch unread count', e);
-	}
-}
-
-function optimisticMarkRead(id: string): void {
-	const notification = notifications.find((n) => n.id === id);
-	if (notification && !notification.read) {
-		notification.read = true;
-		unreadCount = Math.max(0, unreadCount - 1);
-	}
-}
-
-function optimisticMarkAllRead(): void {
-	for (const n of notifications) {
-		n.read = true;
-	}
-	unreadCount = 0;
-}
-
-function init(): void {
-	refCount += 1;
-	if (refCount > 1) return;
-
-	fetchUnreadCount();
-	eventSource = new EventSource('/api/notifications/stream');
-	eventSource.addEventListener('notification', (e) => {
-		const notification = parseNotificationEvent((e as MessageEvent).data);
-		if (!notification) return;
-		notifications = [notification, ...notifications];
-		if (!notification.read) unreadCount += 1;
-	});
-	eventSource.onopen = () => {
-		fetchUnreadCount();
-		errorCount = 0;
+export function createNotificationStore(): NotificationStore {
+	let notifications = $state<NotificationItem[]>([]);
+	let unreadCount = $state(0);
+	let eventSource: EventSource | null = null;
+	let refCount = 0;
+	let errorCount = 0;
+	let navigate: (url: string) => void = (url) => {
+		window.location.href = url;
 	};
-	eventSource.onerror = async () => {
-		if (eventSource?.readyState === EventSource.CLOSED) {
-			const res = await fetch('/api/notifications/unread-count');
-			if (res.status === 401) navigate('/login');
-			return;
+
+	async function fetchNotifications(): Promise<void> {
+		try {
+			const res = await fetch('/api/notifications?size=10');
+			if (res.ok) {
+				const data = await res.json();
+				notifications = data.content ?? [];
+			}
+		} catch (e) {
+			console.error('Failed to fetch notifications', e);
 		}
-		errorCount += 1;
-		if (errorCount >= 3) {
+	}
+
+	async function fetchUnreadCount(): Promise<void> {
+		try {
+			const res = await fetch('/api/notifications/unread-count');
+			if (res.ok) {
+				const data = await res.json();
+				unreadCount = data.count;
+			}
+		} catch (e) {
+			console.error('Failed to fetch unread count', e);
+		}
+	}
+
+	function optimisticMarkRead(id: string): void {
+		const notification = notifications.find((n) => n.id === id);
+		if (notification && !notification.read) {
+			notification.read = true;
+			unreadCount = Math.max(0, unreadCount - 1);
+		}
+	}
+
+	function optimisticMarkAllRead(): void {
+		for (const n of notifications) {
+			n.read = true;
+		}
+		unreadCount = 0;
+	}
+
+	function init(): void {
+		refCount += 1;
+		if (refCount > 1) return;
+
+		fetchUnreadCount();
+		eventSource = new EventSource('/api/notifications/stream');
+		eventSource.addEventListener('notification', (e) => {
+			const notification = parseNotificationEvent((e as MessageEvent).data);
+			if (!notification) return;
+			notifications = [notification, ...notifications];
+			if (!notification.read) unreadCount += 1;
+		});
+		eventSource.onopen = () => {
+			fetchUnreadCount();
+			errorCount = 0;
+		};
+		eventSource.onerror = async () => {
+			if (eventSource?.readyState === EventSource.CLOSED) {
+				const res = await fetch('/api/notifications/unread-count');
+				if (res.status === 401) navigate('/login');
+				return;
+			}
+			errorCount += 1;
+			if (errorCount >= 3) {
+				eventSource?.close();
+				eventSource = null;
+				errorCount = 0;
+				const res = await fetch('/api/notifications/unread-count');
+				if (res.status === 401) navigate('/login');
+			}
+		};
+	}
+
+	function destroy(): void {
+		if (refCount === 0) return;
+		refCount -= 1;
+		if (refCount === 0) {
 			eventSource?.close();
 			eventSource = null;
-			errorCount = 0;
-			const res = await fetch('/api/notifications/unread-count');
-			if (res.status === 401) navigate('/login');
+		}
+	}
+
+	return {
+		get notifications() {
+			return notifications;
+		},
+		get unreadCount() {
+			return unreadCount;
+		},
+		fetchNotifications,
+		fetchUnreadCount,
+		optimisticMarkRead,
+		optimisticMarkAllRead,
+		init,
+		destroy,
+		setNotifications(items: NotificationItem[]): void {
+			notifications = items;
+			unreadCount = items.filter((n) => !n.read).length;
+		},
+		setNavigate(fn: (url: string) => void): void {
+			navigate = fn;
 		}
 	};
 }
 
-function destroy(): void {
-	if (refCount === 0) return;
-	refCount -= 1;
-	if (refCount === 0) {
-		eventSource?.close();
-		eventSource = null;
+/**
+ * Create a notification store and put it on the context. Call once high in the
+ * tree (root +layout.svelte). Descendants read it via getNotificationStore().
+ */
+export function provideNotificationStore(): NotificationStore {
+	const store = createNotificationStore();
+	setContext(NOTIFICATION_KEY, store);
+	return store;
+}
+
+export function getNotificationStore(): NotificationStore {
+	let store: NotificationStore | undefined;
+	try {
+		store = getContext<NotificationStore>(NOTIFICATION_KEY);
+	} catch {
+		throw new Error(
+			'NotificationStore not found — call provideNotificationStore() in +layout.svelte'
+		);
 	}
+	if (!store)
+		throw new Error(
+			'NotificationStore not found — call provideNotificationStore() in +layout.svelte'
+		);
+	return store;
 }
-
-export function __resetForTest(): void {
-	eventSource?.close();
-	eventSource = null;
-	refCount = 0;
-	errorCount = 0;
-	notifications = [];
-	unreadCount = 0;
-}
-
-export function __setNavigateForTest(fn: (url: string) => void): void {
-	navigate = fn;
-}
-
-export const notificationStore = {
-	get notifications() {
-		return notifications;
-	},
-	get unreadCount() {
-		return unreadCount;
-	},
-	fetchNotifications,
-	fetchUnreadCount,
-	optimisticMarkRead,
-	optimisticMarkAllRead,
-	init,
-	destroy
-};