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/App.tsx

@@ -1,6 +1,8 @@
 import { QueryClient, QueryClientProvider } from '@tanstack/react-query';
 import { BrowserRouter } from 'react-router';
+import { ConfigProvider, theme } from 'antd';
 import { AppRoutes } from '@/routes';
+import { ThemeProvider, useTheme } from '@/contexts/ThemeContext';
 
 const queryClient = new QueryClient({
   defaultOptions: {
@@ -12,12 +14,28 @@ const queryClient = new QueryClient({
   },
 });
 
-function App() {
+function ThemedApp() {
+  const { mode } = useTheme();
+
   return (
-    <QueryClientProvider client={queryClient}>
+    <ConfigProvider
+      theme={{
+        algorithm: mode === 'dark' ? theme.darkAlgorithm : theme.defaultAlgorithm,
+      }}
+    >
       <BrowserRouter>
         <AppRoutes />
       </BrowserRouter>
+    </ConfigProvider>
+  );
+}
+
+function App() {
+  return (
+    <QueryClientProvider client={queryClient}>
+      <ThemeProvider>
+        <ThemedApp />
+      </ThemeProvider>
     </QueryClientProvider>
   );
 }