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/pages/Chat.tsx

@@ -4,9 +4,13 @@
 
 import { useEffect, useRef } from 'react';
 import { Wifi, WifiOff } from 'lucide-react';
-import { useChat } from '../hooks/useChat';
-import { ChatMessage, StreamingMessage, TypingIndicator } from '../components/ChatMessage';
-import { ChatInput } from '../components/ChatInput';
+import {
+  useChat,
+  ChatMessage,
+  StreamingMessage,
+  TypingIndicator,
+  ChatInput,
+} from '@ai-assistant/ui';
 
 interface ChatPageProps {
   sessionId: string;