Refactor admin stats API and charts data models for consistency

- Updated `AdminStatsResponse` with streamlined type annotations and added `AdminStatsData` type definition.
- Renamed chart data model fields (`totalUsers` → `total_users`, `activeUsers` → `active_users`, `members` → `value`, etc.) for alignment with backend naming conventions.
- Adjusted related test files to reflect updated data model structure.
- Improved readability of `AdminPage` component by reformatting destructuring in `useQuery`.

frontend/src/app/[locale]/admin/page.tsx

@@ -19,7 +19,11 @@ import { useQuery } from '@tanstack/react-query';
 import { getAdminStats } from '@/lib/api/admin';
 
 export default function AdminPage() {
-  const { data: stats, isLoading, error } = useQuery({
+  const {
+    data: stats,
+    isLoading,
+    error,
+  } = useQuery({
     queryKey: ['admin', 'stats'],
     queryFn: async () => {
       const response = await getAdminStats();