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/components/dashboard/AISummaryCard.tsx

@@ -9,7 +9,9 @@ interface Props {
 
 function generateSummary(data: DashboardData): string {
   const parts: string[] = [];
-  const { deadline_summary: ds, case_summary: cs, upcoming_deadlines: ud } = data;
+  const ds = data.deadline_summary ?? { overdue_count: 0, due_this_week: 0, due_next_week: 0, ok_count: 0 };
+  const cs = data.case_summary ?? { active_count: 0, new_this_month: 0, closed_count: 0 };
+  const ud = Array.isArray(data.upcoming_deadlines) ? data.upcoming_deadlines : [];
 
   // Deadline urgency
   if (ds.overdue_count > 0) {