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/app/(app)/einstellungen/page.tsx

@@ -22,7 +22,7 @@ export default function EinstellungenPage() {
     refetch,
   } = useQuery({
     queryKey: ["tenant-current", tenantId],
-    queryFn: () => api.get<Tenant>(`/api/tenants/${tenantId}`),
+    queryFn: () => api.get<Tenant>(`/tenants/${tenantId}`),
     enabled: !!tenantId,
   });