test(notifications): add SSE event handling tests for useNotificationStream

Adds MockEventSource.simulate() helper and two tests covering:
- unread notification via SSE prepends to list and increments unreadCount
- read notification via SSE adds to list but does not increment unreadCount

Fixes @saraholt: "SSE event handling not tested"

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/lib/hooks/__tests__/useNotificationStream.svelte.test.ts

@@ -6,6 +6,7 @@ let lastEventSource: {
 	close: ReturnType<typeof vi.fn>;
 	onopen: (() => void) | null;
 	onerror: (() => void) | null;
+	simulate: (type: string, data: string) => void;
 } | null = null;
 
 class MockEventSource {
@@ -23,6 +24,13 @@ class MockEventSource {
 		if (!this.listeners[type]) this.listeners[type] = [];
 		this.listeners[type].push(fn);
 	}
+
+	simulate(type: string, data: string) {
+		const event = new MessageEvent(type, { data });
+		for (const fn of this.listeners[type] ?? []) {
+			fn(event);
+		}
+	}
 }
 
 vi.stubGlobal('EventSource', MockEventSource);
@@ -106,4 +114,29 @@ describe('createNotificationStream', () => {
 		stream.destroy();
 		expect(lastEventSource!.close).toHaveBeenCalled();
 	});
+
+	it('SSE notification event prepends notification and increments unreadCount', async () => {
+		mockFetch.mockResolvedValue(new Response(JSON.stringify({ count: 0 }), { status: 200 }));
+		const stream = createNotificationStream();
+		stream.init();
+
+		const notification = makeNotification({ id: 'sse-1', read: false });
+		lastEventSource!.simulate('notification', JSON.stringify(notification));
+
+		expect(stream.notifications).toHaveLength(1);
+		expect(stream.notifications[0].id).toBe('sse-1');
+		expect(stream.unreadCount).toBe(1);
+	});
+
+	it('SSE notification event with read:true does not increment unreadCount', async () => {
+		mockFetch.mockResolvedValue(new Response(JSON.stringify({ count: 0 }), { status: 200 }));
+		const stream = createNotificationStream();
+		stream.init();
+
+		const notification = makeNotification({ id: 'sse-2', read: true });
+		lastEventSource!.simulate('notification', JSON.stringify(notification));
+
+		expect(stream.notifications).toHaveLength(1);
+		expect(stream.unreadCount).toBe(0);
+	});
 });