Fix admin login API endpoint: Use Next.js proxy instead of direct backend call

- Changed from process.env API_URL to /api/auth/login
- Fixes network error by using Next.js API rewrites
- Login now properly routes through frontend proxy to backend

frontend/src/app/admin/page.tsx

@@ -18,8 +18,7 @@ export default function AdminLogin() {
     setError('');
 
     try {
-      const apiUrl = process.env['NEXT_PUBLIC_API_URL'] || 'http://localhost:3001/api';
-      const response = await fetch(`${apiUrl}/auth/login`, {
+      const response = await fetch('/api/auth/login', {
         method: 'POST',
         headers: {
           'Content-Type': 'application/json',