feat: 升级 Ant Design 6 并实现主题切换功能

- 升级 antd 从 5.24.9 到 6.3.5，@ant-design/icons 从 5.6.1 到 6.1.1
- 新增 ThemeContext 和 ThemeToggle 组件，支持明暗主题切换
- 移除自定义 SCSS 样式，采用 Ant Design 主题系统
- 测试环境从 jsdom 切换到 happy-dom，提升测试性能
- 更新 AppLayout、ModelForm、ProviderForm 以适配新主题系统

frontend/src/components/AppLayout/index.tsx

@@ -1,10 +1,7 @@
 import { Layout, Menu } from 'antd';
-import {
-  CloudServerOutlined,
-  BarChartOutlined,
-} from '@ant-design/icons';
+import { CloudServerOutlined, BarChartOutlined } from '@ant-design/icons';
 import { Outlet, useLocation, useNavigate } from 'react-router';
-import styles from './AppLayout.module.scss';
+import { ThemeToggle } from '@/components/ThemeToggle';
 
 const menuItems = [
   { key: '/providers', label: '供应商管理', icon: <CloudServerOutlined /> },
@@ -16,19 +13,44 @@ export function AppLayout() {
   const navigate = useNavigate();
 
   return (
-    <Layout className={styles.layout}>
-      <Layout.Header className={styles.header}>
-        <div className={styles.logo}>AI Gateway</div>
+    <Layout style={{ minHeight: '100vh' }}>
+      <Layout.Header
+        style={{
+          display: 'flex',
+          alignItems: 'center',
+          padding: '0 2rem',
+        }}
+      >
+        <div
+          style={{
+            color: '#fff',
+            fontSize: '1.25rem',
+            fontWeight: 600,
+            marginRight: '2rem',
+            whiteSpace: 'nowrap',
+          }}
+        >
+          AI Gateway
+        </div>
         <Menu
           theme="dark"
           mode="horizontal"
           selectedKeys={[location.pathname]}
           items={menuItems}
           onClick={({ key }) => navigate(key)}
-          className={styles.menu}
+          style={{ flex: 1, minWidth: 0 }}
         />
+        <ThemeToggle />
       </Layout.Header>
-      <Layout.Content className={styles.content}>
+      <Layout.Content
+        style={{
+          padding: '2rem',
+          maxWidth: '1400px',
+          width: '100%',
+          margin: '0 auto',
+          boxSizing: 'border-box',
+        }}
+      >
         <Outlet />
       </Layout.Content>
     </Layout>