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)/fristen/page.tsx

@@ -66,7 +66,7 @@ export default function FristenPage() {
       {view === "list" ? (
         <DeadlineList />
       ) : (
-        <DeadlineCalendarView deadlines={deadlines || []} />
+        <DeadlineCalendarView deadlines={Array.isArray(deadlines) ? deadlines : []} />
       )}
     </div>
   );