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/DeadlineTrafficLights.tsx

@@ -31,24 +31,25 @@ interface Props {
 }
 
 export function DeadlineTrafficLights({ data, onFilter }: Props) {
+  const safe = data ?? { overdue_count: 0, due_this_week: 0, due_next_week: 0, ok_count: 0 };
   const cards = [
     {
       key: "overdue" as const,
       label: "Überfällig",
-      count: data.overdue_count,
+      count: safe.overdue_count ?? 0,
       icon: AlertTriangle,
       bg: "bg-red-50",
       border: "border-red-200",
       iconColor: "text-red-500",
       countColor: "text-red-700",
       labelColor: "text-red-600",
-      ring: data.overdue_count > 0 ? "ring-2 ring-red-300 ring-offset-1" : "",
-      pulse: data.overdue_count > 0,
+      ring: (safe.overdue_count ?? 0) > 0 ? "ring-2 ring-red-300 ring-offset-1" : "",
+      pulse: (safe.overdue_count ?? 0) > 0,
     },
     {
       key: "this_week" as const,
       label: "Diese Woche",
-      count: data.due_this_week,
+      count: safe.due_this_week ?? 0,
       icon: Clock,
       bg: "bg-amber-50",
       border: "border-amber-200",
@@ -61,7 +62,7 @@ export function DeadlineTrafficLights({ data, onFilter }: Props) {
     {
       key: "ok" as const,
       label: "Im Zeitplan",
-      count: data.ok_count + data.due_next_week,
+      count: (safe.ok_count ?? 0) + (safe.due_next_week ?? 0),
       icon: CheckCircle,
       bg: "bg-emerald-50",
       border: "border-emerald-200",