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/package.json

@@ -14,9 +14,9 @@
     "test:e2e": "playwright test"
   },
   "dependencies": {
-    "@ant-design/icons": "^5.6.1",
+    "@ant-design/icons": "^6.1.1",
     "@tanstack/react-query": "^5.80.2",
-    "antd": "^5.24.9",
+    "antd": "^6.3.5",
     "react": "^19.2.4",
     "react-dom": "^19.2.4",
     "react-router": "^7.6.1"
@@ -38,6 +38,7 @@
     "eslint-plugin-react-hooks": "^7.0.1",
     "eslint-plugin-react-refresh": "^0.5.2",
     "globals": "^17.4.0",
+    "happy-dom": "^20.9.0",
     "jsdom": "^26.1.0",
     "msw": "^2.8.2",
     "sass": "^1.99.0",