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)/termine/page.tsx

@@ -84,7 +84,7 @@ export default function TerminePage() {
         <AppointmentList onEdit={handleEdit} />
       ) : (
         <AppointmentCalendar
-          appointments={appointments || []}
+          appointments={Array.isArray(appointments) ? appointments : []}
           onAppointmentClick={handleEdit}
         />
       )}