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/CaseOverviewGrid.tsx

@@ -8,24 +8,25 @@ interface Props {
 }
 
 export function CaseOverviewGrid({ data }: Props) {
+  const safe = data ?? { active_count: 0, new_this_month: 0, closed_count: 0 };
   const items = [
     {
       label: "Aktive Akten",
-      value: data.active_count,
+      value: safe.active_count ?? 0,
       icon: FolderOpen,
       color: "text-blue-600",
       bg: "bg-blue-50",
     },
     {
       label: "Neu (Monat)",
-      value: data.new_this_month,
+      value: safe.new_this_month ?? 0,
       icon: FolderPlus,
       color: "text-violet-600",
       bg: "bg-violet-50",
     },
     {
       label: "Abgeschlossen",
-      value: data.closed_count,
+      value: safe.closed_count ?? 0,
       icon: Archive,
       color: "text-neutral-500",
       bg: "bg-neutral-50",