fix(auth): improve login error feedback for better UX

src/pages/Login.tsx

@@ -1,4 +1,4 @@
-import { useState } from 'react';
+import { useState, useEffect } from 'react';
 import { Form, Input, Button, Card, message, Checkbox } from 'antd';
 import { UserOutlined, LockOutlined, LoginOutlined } from '@ant-design/icons';
 import { useNavigate } from 'react-router-dom';
@@ -12,6 +12,16 @@ function LoginPage() {
   const [loading, setLoading] = useState(false);
   const navigate = useNavigate();
 
+  useEffect(() => {
+    const token = localStorage.getItem('authToken');
+    if (token) {
+      const currentUser = authApi.getCurrentUser();
+      if (currentUser) {
+        navigate('/admin/dashboard', { replace: true });
+      }
+    }
+  }, [navigate]);
+
   const handleLogin = async (values: { username: string; password: string; remember?: boolean }) => {
     setLoading(true);
     try {