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/deadlines/DeadlineList.tsx

@@ -76,12 +76,12 @@ export function DeadlineList() {
 
   const caseMap = useMemo(() => {
     const map = new Map<string, Case>();
-    cases?.forEach((c) => map.set(c.id, c));
+    (Array.isArray(cases) ? cases : []).forEach((c) => map.set(c.id, c));
     return map;
   }, [cases]);
 
   const filtered = useMemo(() => {
-    if (!deadlines) return [];
+    if (!Array.isArray(deadlines)) return [];
     return deadlines.filter((d) => {
       if (statusFilter === "pending" && d.status !== "pending") return false;
       if (statusFilter === "completed" && d.status !== "completed")
@@ -96,7 +96,7 @@ export function DeadlineList() {
   }, [deadlines, statusFilter, caseFilter]);
 
   const counts = useMemo(() => {
-    if (!deadlines) return { overdue: 0, thisWeek: 0, ok: 0 };
+    if (!Array.isArray(deadlines)) return { overdue: 0, thisWeek: 0, ok: 0 };
     let overdue = 0,
       thisWeek = 0,
       ok = 0;
@@ -188,7 +188,7 @@ export function DeadlineList() {
           <option value="completed">Erledigt</option>
           <option value="overdue">Überfällig</option>
         </select>
-        {cases && cases.length > 0 && (
+        {Array.isArray(cases) && cases.length > 0 && (
           <select
             value={caseFilter}
             onChange={(e) => setCaseFilter(e.target.value)}