fix: remove remaining /api/ double-prefix from template literal API calls

Previous fix missed backtick template strings. Fixed 7 more api.*()
calls in appointments, deadlines, settings, and einstellungen pages.

frontend/src/components/appointments/AppointmentModal.tsx

@@ -78,7 +78,7 @@ export function AppointmentModal({ open, onClose, appointment }: AppointmentModa
 
   const updateMutation = useMutation({
     mutationFn: (body: Record<string, unknown>) =>
-      api.put<Appointment>(`/api/appointments/${appointment!.id}`, body),
+      api.put<Appointment>(`/appointments/${appointment!.id}`, body),
     onSuccess: () => {
       queryClient.invalidateQueries({ queryKey: ["appointments"] });
       queryClient.invalidateQueries({ queryKey: ["dashboard"] });