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/deadlines/DeadlineList.tsx

@@ -64,7 +64,7 @@ export function DeadlineList() {
 
   const completeMutation = useMutation({
     mutationFn: (id: string) =>
-      api.patch<Deadline>(`/api/deadlines/${id}/complete`),
+      api.patch<Deadline>(`/deadlines/${id}/complete`),
     onSuccess: () => {
       queryClient.invalidateQueries({ queryKey: ["deadlines"] });
       toast.success("Frist als erledigt markiert");