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/activity/ChronikFuerDichBox.svelte

@@ -81,8 +81,8 @@ function href(n: NotificationItem): string {
 					return async ({ result, update }) => {
 						if (result.type === 'failure' || result.type === 'error') {
 							errorMessage = m.notification_error_generic();
+							await update({ reset: false, invalidateAll: false });
 						}
-						await update({ reset: false, invalidateAll: false });
 					};
 				}}
 			>
@@ -129,8 +129,8 @@ function href(n: NotificationItem): string {
 							return async ({ result, update }) => {
 								if (result.type === 'failure' || result.type === 'error') {
 									errorMessage = m.notification_error_generic();
+									await update({ reset: false, invalidateAll: false });
 								}
-								await update({ reset: false, invalidateAll: false });
 							};
 						}}
 					>

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, {

frontend/src/routes/aktivitaeten/+page.server.ts

@@ -73,7 +73,7 @@ export const actions = {
 		const data = await request.formData();
 		const raw = data.get('notificationId');
 		const notificationId = typeof raw === 'string' ? raw : null;
-		if (!notificationId) return fail(400, { error: null });
+		if (!notificationId) return fail(400, { error: getErrorMessage(undefined) });
 		const api = createApiClient(fetch);
 		const result = await api.PATCH('/api/notifications/{id}/read', {
 			params: { path: { id: notificationId } }