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)/cases/page.tsx

@@ -68,7 +68,7 @@ export default function CasesPage() {
     },
   });
 
-  const cases = data?.cases ?? [];
+  const cases = Array.isArray(data?.cases) ? data.cases : [];
 
   return (
     <div className="animate-fade-in">