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/layout/TenantSwitcher.tsx

@@ -15,6 +15,7 @@ export function TenantSwitcher() {
     api
       .get<TenantWithRole[]>("/tenants")
       .then((data) => {
+        if (!Array.isArray(data)) return;
         setTenants(data);
         const savedId = localStorage.getItem("kanzlai_tenant_id");
         const match = data.find((t) => t.id === savedId) || data[0];