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/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 } }