如何在Flutter中实现这个UI？

import 'package:flutter/material.dart';

void main() {
  runApp(LoginApp());
}

class LoginApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Login App',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: LoginScreen(),
    );
  }
}

class LoginScreen extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Login'),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            TextField(
              decoration: InputDecoration(
                labelText: 'Username',
              ),
            ),
            SizedBox(height: 20),
            TextField(
              decoration: InputDecoration(
                labelText: 'Password',
              ),
              obscureText: true,
            ),
            SizedBox(height: 20),
            ElevatedButton(
              onPressed: () {
                // 登录逻辑
              },
              child: Text('Login'),
            ),
          ],
        ),
      ),
    );
  }
}