feat(ui): 添加文件 Diff 查看功能

当 AI 执行 write_file 或 edit_file 工具时，在工具结果中显示 View Diff 按钮，
点击后在 IDE 面板中显示文件修改前后的对比视图。

主要改动：
- core: edit_file/write_file 工具返回 fileDiff 元数据
- ui: 新增 DiffEditor 组件用于显示文件差异
- ui: ChatMessage 添加 View Diff 按钮
- ui: IDE 组件支持 Diff 视图切换
- ui: useChat hook 处理 fileDiff 回调

packages/web/src/App.tsx

@@ -24,6 +24,7 @@ import {
   createSession,
   type Session,
   type ActiveFileInfo,
+  type FileDiffInfo,
 } from '@ai-assistant/ui';
 import { ChatPage } from './pages/Chat';
 
@@ -53,6 +54,9 @@ export function App() {
     return saved !== 'false'; // 默认开启
   });
 
+  // Diff 显示状态（当 AI 编辑/写入文件时触发）
+  const [pendingDiff, setPendingDiff] = useState<FileDiffInfo | null>(null);
+
   // 持久化自动附加开关状态
   useEffect(() => {
     localStorage.setItem('ai-assistant-auto-attach-file', String(autoAttachActiveFile));
@@ -121,6 +125,16 @@ export function App() {
     setCurrentSessionId(newSessionId);
   }, []);
 
+  // 文件 diff 回调（当 AI 编辑/写入文件时触发）
+  const handleFileDiff = useCallback((diff: FileDiffInfo) => {
+    setPendingDiff(diff);
+  }, []);
+
+  // Diff 关闭回调
+  const handleDiffClose = useCallback(() => {
+    setPendingDiff(null);
+  }, []);
+
   // 处理面板宽度调整
   const handleResize = useCallback((delta: number) => {
     setIdePanelWidth((prev) => {
@@ -159,7 +173,11 @@ export function App() {
             className="hidden md:flex flex-col"
             style={{ width: `${idePanelWidth}%` }}
           >
-            <IDE onActiveFileChange={setActiveFile} />
+            <IDE
+              onActiveFileChange={setActiveFile}
+              pendingDiff={pendingDiff}
+              onDiffClose={handleDiffClose}
+            />
           </div>
 
           {/* 可拖拽分割线 */}
@@ -191,6 +209,8 @@ export function App() {
                 activeFile={activeFile}
                 autoAttachActiveFile={autoAttachActiveFile}
                 onAutoAttachActiveFileToggle={setAutoAttachActiveFile}
+                onFileDiff={handleFileDiff}
+                onViewDiff={handleFileDiff}
               />
             ) : (
               <div className="flex-1 flex items-center justify-center h-full">

packages/web/src/pages/Chat.tsx

@@ -17,6 +17,7 @@ import {
   DiagnosticsIndicator,
   ToolbarOverflowMenu,
   type ActiveFileInfo,
+  type FileDiffInfo,
 } from '@ai-assistant/ui';
 
 interface ChatPageProps {
@@ -43,6 +44,10 @@ interface ChatPageProps {
   autoAttachActiveFile?: boolean;
   /** 自动附加开关变更回调 */
   onAutoAttachActiveFileToggle?: (enabled: boolean) => void;
+  /** 文件 diff 回调（当 AI 写入/编辑文件时触发） */
+  onFileDiff?: (diff: FileDiffInfo) => void;
+  /** 查看文件 diff 回调（点击 View Diff 按钮） */
+  onViewDiff?: (diff: FileDiffInfo) => void;
 }
 
 export function ChatPage({
@@ -63,6 +68,8 @@ export function ChatPage({
   activeFile,
   autoAttachActiveFile,
   onAutoAttachActiveFileToggle,
+  onFileDiff,
+  onViewDiff,
 }: ChatPageProps) {
   const {
     messages,
@@ -100,6 +107,7 @@ export function ChatPage({
           : undefined,
       });
     },
+    onFileDiff,
   });
 
   const messagesEndRef = useRef<HTMLDivElement>(null);
@@ -208,13 +216,13 @@ export function ChatPage({
 
           <AnimatePresence mode="popLayout">
             {messages.map((message) => (
-              <ChatMessage key={message.id} message={message} onAnswerQuestion={answerQuestion} />
+              <ChatMessage key={message.id} message={message} onAnswerQuestion={answerQuestion} onViewDiff={onViewDiff ?? onFileDiff} />
             ))}
           </AnimatePresence>
 
           {/* 流式消息 - 复用 ChatMessage 组件 */}
           {streamingMessage && (
-            <ChatMessage message={streamingMessage} isStreaming onAnswerQuestion={answerQuestion} />
+            <ChatMessage message={streamingMessage} isStreaming onAnswerQuestion={answerQuestion} onViewDiff={onViewDiff ?? onFileDiff} />
           )}
 
           {/* 子 Agent 进度显示 */}