fix: add array guards to all frontend components consuming API responses

Prevents "M.forEach is not a function" crashes when API returns error
objects or unexpected shapes instead of arrays. Guards all useQuery
consumers with Array.isArray checks and safe defaults for object props.

Files fixed: DeadlineList, AppointmentList, TenantSwitcher,
DeadlineTrafficLights, UpcomingTimeline, CaseOverviewGrid,
AISummaryCard, TeamSettings, and all page-level components
(dashboard, cases, fristen, termine, ai/extract).

frontend/src/app/(app)/ai/extract/page.tsx

@@ -27,7 +27,7 @@ export default function AIExtractPage() {
     queryFn: () => api.get<PaginatedResponse<Case>>("/cases"),
   });
 
-  const cases = casesData?.data ?? [];
+  const cases = Array.isArray(casesData?.data) ? casesData.data : [];
 
   async function handleExtract(file: File | null, text: string) {
     setIsExtracting(true);