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/app/(app)/ai/extract/page.tsx

@@ -27,7 +27,7 @@ export default function AIExtractPage() {
     queryFn: () => api.get<PaginatedResponse<Case>>("/cases"),
   });
 
-  const cases = casesData?.data ?? [];
+  const cases = Array.isArray(casesData?.data) ? casesData.data : [];
 
   async function handleExtract(file: File | null, text: string) {
     setIsExtracting(true);

frontend/src/app/(app)/cases/[id]/page.tsx

@@ -132,8 +132,8 @@ export default function CaseDetailPage() {
     );
   }
 
-  const deadlines = deadlinesData?.deadlines ?? [];
-  const documents = documentsData ?? [];
+  const deadlines = Array.isArray(deadlinesData?.deadlines) ? deadlinesData.deadlines : [];
+  const documents = Array.isArray(documentsData) ? documentsData : [];
 
   return (
     <div className="animate-fade-in">
@@ -205,7 +205,7 @@ export default function CaseDetailPage() {
                   {caseDetail.deadlines_count}
                 </span>
               )}
-              {tab.key === "parties" && caseDetail.parties.length > 0 && (
+              {tab.key === "parties" && Array.isArray(caseDetail.parties) && caseDetail.parties.length > 0 && (
                 <span className="ml-1 rounded-full bg-neutral-100 px-1.5 py-0.5 text-xs text-neutral-500">
                   {caseDetail.parties.length}
                 </span>
@@ -217,7 +217,7 @@ export default function CaseDetailPage() {
 
       <div className="mt-6">
         {activeTab === "timeline" && (
-          <CaseTimeline events={caseDetail.recent_events ?? []} />
+          <CaseTimeline events={Array.isArray(caseDetail.recent_events) ? caseDetail.recent_events : []} />
         )}
 
         {activeTab === "deadlines" && (
@@ -229,7 +229,7 @@ export default function CaseDetailPage() {
         )}
 
         {activeTab === "parties" && (
-          <PartyList caseId={id} parties={caseDetail.parties ?? []} />
+          <PartyList caseId={id} parties={Array.isArray(caseDetail.parties) ? caseDetail.parties : []} />
         )}
       </div>
     </div>

frontend/src/app/(app)/cases/page.tsx

@@ -68,7 +68,7 @@ export default function CasesPage() {
     },
   });
 
-  const cases = data?.cases ?? [];
+  const cases = Array.isArray(data?.cases) ? data.cases : [];
 
   return (
     <div className="animate-fade-in">

frontend/src/app/(app)/dashboard/page.tsx

@@ -80,17 +80,17 @@ export default function DashboardPage() {
         </p>
       </div>
 
-      <DeadlineTrafficLights data={data.deadline_summary} />
+      <DeadlineTrafficLights data={data.deadline_summary ?? { overdue_count: 0, due_this_week: 0, due_next_week: 0, ok_count: 0 }} />
 
       <div className="grid grid-cols-1 gap-6 lg:grid-cols-3">
         <div className="lg:col-span-2">
           <UpcomingTimeline
-            deadlines={data.upcoming_deadlines}
-            appointments={data.upcoming_appointments}
+            deadlines={Array.isArray(data.upcoming_deadlines) ? data.upcoming_deadlines : []}
+            appointments={Array.isArray(data.upcoming_appointments) ? data.upcoming_appointments : []}
           />
         </div>
         <div className="space-y-6">
-          <CaseOverviewGrid data={data.case_summary} />
+          <CaseOverviewGrid data={data.case_summary ?? { active_count: 0, new_this_month: 0, closed_count: 0 }} />
           <AISummaryCard data={data} />
           <QuickActions />
         </div>

frontend/src/app/(app)/fristen/page.tsx

@@ -66,7 +66,7 @@ export default function FristenPage() {
       {view === "list" ? (
         <DeadlineList />
       ) : (
-        <DeadlineCalendarView deadlines={deadlines || []} />
+        <DeadlineCalendarView deadlines={Array.isArray(deadlines) ? deadlines : []} />
       )}
     </div>
   );

frontend/src/app/(app)/termine/page.tsx

@@ -84,7 +84,7 @@ export default function TerminePage() {
         <AppointmentList onEdit={handleEdit} />
       ) : (
         <AppointmentCalendar
-          appointments={appointments || []}
+          appointments={Array.isArray(appointments) ? appointments : []}
           onAppointmentClick={handleEdit}
         />
       )}