perf(auth): eliminate login page flicker by moving session check to App.tsx

src/pages/Login.tsx

@@ -1,4 +1,4 @@
-import { useState, useEffect } from 'react';
+import { useState } 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';
@@ -12,16 +12,6 @@ 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 {