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/UpcomingTimeline.tsx

@@ -21,13 +21,16 @@ function formatDayLabel(date: Date): string {
 }
 
 export function UpcomingTimeline({ deadlines, appointments }: Props) {
+  const safeDeadlines = Array.isArray(deadlines) ? deadlines : [];
+  const safeAppointments = Array.isArray(appointments) ? appointments : [];
+
   const items: TimelineItem[] = [
-    ...deadlines.map((d) => ({
+    ...safeDeadlines.map((d) => ({
       type: "deadline" as const,
       date: parseISO(d.due_date),
       data: d,
     })),
-    ...appointments.map((a) => ({
+    ...safeAppointments.map((a) => ({
       type: "appointment" as const,
       date: parseISO(a.start_at),
       data: a,