fix(notification): address review suggestions

- ChronikFuerDichBox: move update() inside the failure branch so success
  path skips it, matching NotificationDropdown's pattern
- NotificationDropdown test: add role=alert assertion for mark-all-read
  failure to match existing dismiss-failure coverage in ChronikFuerDichBox
- +page.server.ts: use getErrorMessage(undefined) instead of null so the
  missing-notificationId 400 goes through the same i18n pipeline as other errors

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

frontend/src/lib/notification/NotificationDropdown.svelte.test.ts

@@ -234,6 +234,23 @@ describe('NotificationDropdown', () => {
 		expect(optimisticMarkAllRead).toHaveBeenCalledOnce();
 	});
 
+	it('shows a role=alert error banner when mark-all-read returns a failure', async () => {
+		mockFormResult.type = 'failure';
+		render(NotificationDropdown, {
+			props: {
+				notifications: [makeNotification()],
+				optimisticMarkRead: () => {},
+				optimisticMarkAllRead: () => {},
+				onClose: () => {}
+			}
+		});
+
+		await page.getByRole('button', { name: /alle gelesen/i }).click();
+
+		const alert = document.querySelector('[role="alert"]');
+		expect(alert).not.toBeNull();
+	});
+
 	it('calls onClose when the view-all button is clicked', async () => {
 		const onClose = vi.fn();
 		render(NotificationDropdown, {