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/src/App.tsx

@@ -3,11 +3,15 @@
  */
 
 import { useState, useEffect } from 'react';
-import { Sidebar } from './components/Sidebar';
+import {
+  Sidebar,
+  FileBrowser,
+  ConfigPanel,
+  listSessions,
+  createSession,
+  type Session,
+} from '@ai-assistant/ui';
 import { ChatPage } from './pages/Chat';
-import { FileBrowser } from './components/FileBrowser';
-import { ConfigPanel } from './components/ConfigPanel';
-import { listSessions, createSession, type Session } from './api/client';
 
 export function App() {
   const [currentSessionId, setCurrentSessionId] = useState<string | null>(null);