refactor(notification): rename markRead/markAllRead to optimistic helpers without fetch

Removes raw fetch() calls from the store. optimisticMarkRead(id) and
optimisticMarkAllRead() now only mutate local $state — the actual API
calls move to SvelteKit form actions on /aktivitaeten.

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

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

@@ -108,12 +108,46 @@ describe('notificationStore (singleton)', () => {
 		expect(notificationStore.unreadCount).toBe(1);
 	});
 
-	it('markAllRead resets unreadCount', async () => {
-		mockFetch.mockResolvedValue(new Response(null, { status: 200 }));
-		await notificationStore.markAllRead();
+	it('optimisticMarkRead marks the notification read and decrements unreadCount without fetching', () => {
+		notificationStore.init();
+		const notification = makeNotification({ id: 'sse-1', read: false });
+		lastEventSource!.simulate('notification', JSON.stringify(notification));
+		mockFetch.mockReset(); // clear the fetchUnreadCount call from init
 
-		expect(mockFetch).toHaveBeenCalledWith('/api/notifications/read-all', { method: 'POST' });
+		notificationStore.optimisticMarkRead('sse-1');
+
+		expect(notificationStore.notifications[0].read).toBe(true);
 		expect(notificationStore.unreadCount).toBe(0);
+		expect(mockFetch).not.toHaveBeenCalled();
+	});
+
+	it('optimisticMarkRead on an already-read notification does not decrement unreadCount below 0', () => {
+		notificationStore.init();
+		const notification = makeNotification({ id: 'sse-1', read: true });
+		lastEventSource!.simulate('notification', JSON.stringify(notification));
+
+		notificationStore.optimisticMarkRead('sse-1');
+
+		expect(notificationStore.unreadCount).toBe(0);
+	});
+
+	it('optimisticMarkAllRead resets unreadCount and marks all notifications read without fetching', () => {
+		notificationStore.init();
+		lastEventSource!.simulate(
+			'notification',
+			JSON.stringify(makeNotification({ id: 'n1', read: false }))
+		);
+		lastEventSource!.simulate(
+			'notification',
+			JSON.stringify(makeNotification({ id: 'n2', read: false }))
+		);
+		mockFetch.mockReset();
+
+		notificationStore.optimisticMarkAllRead();
+
+		expect(notificationStore.unreadCount).toBe(0);
+		expect(notificationStore.notifications.every((n) => n.read)).toBe(true);
+		expect(mockFetch).not.toHaveBeenCalled();
 	});
 });