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/__tests__/setup.ts

@@ -1,5 +1,10 @@
 import '@testing-library/jest-dom/vitest';
 
+// Ensure jsdom environment is properly initialized
+if (typeof window === 'undefined' || typeof document === 'undefined') {
+  throw new Error('jsdom environment not initialized. Check vitest config.');
+}
+
 // Polyfill window.matchMedia for jsdom (required by antd)
 Object.defineProperty(window, 'matchMedia', {
   writable: true,
@@ -24,3 +29,11 @@ window.getComputedStyle = (elt: Element, pseudoElt?: string | null) => {
     return {} as CSSStyleDeclaration;
   }
 };
+
+// Polyfill ResizeObserver for antd
+global.ResizeObserver = class ResizeObserver {
+  observe() {}
+  unobserve() {}
+  disconnect() {}
+};
+