feat(ui): 创建共享 UI 组件包

将 web 和 desktop 的重复代码抽取到 @ai-assistant/ui 包:

- 添加可配置的 API 客户端 (configureApiClient)
- 迁移共享组件: ChatMessage, ChatInput, Sidebar, FileBrowser, ConfigPanel
- 迁移共享 hook: useChat
- 添加 responsive prop 支持响应式布局
- 更新 web/desktop 依赖并删除重复代码

packages/desktop/package.json

@@ -13,6 +13,7 @@
     "tauri": "tauri"
   },
   "dependencies": {
+    "@ai-assistant/ui": "workspace:*",
     "@tauri-apps/api": "^2.1.1",
     "@tauri-apps/plugin-dialog": "^2.2.0",
     "@tauri-apps/plugin-fs": "^2.2.0",