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/core/src/tools/filesystem/edit_file.ts

@@ -123,9 +123,22 @@ export const editFileTool: ToolWithMetadata = {
       output += `\n\n⚠️ 代码检查发现问题，请修复:${result.diagnostics}`;
     }
 
+    // 构建 diff 元数据
+    const fileName = path.basename(absolutePath);
+    const metadata: Record<string, unknown> = {
+      fileDiff: {
+        path: absolutePath,
+        name: fileName,
+        originalContent: result.originalContent ?? '',
+        newContent: result.newContent ?? '',
+        operation: 'edit',
+      },
+    };
+
     return {
       success: true,
       output,
+      metadata,
     };
   },
 };

packages/core/src/tools/filesystem/write_file.ts

@@ -92,9 +92,22 @@ export const writeFileTool: ToolWithMetadata = {
       output += `\n\n⚠️ 代码检查发现问题，请修复:${result.diagnostics}`;
     }
 
+    // 构建 diff 元数据
+    const fileName = path.basename(absolutePath);
+    const metadata: Record<string, unknown> = {
+      fileDiff: {
+        path: absolutePath,
+        name: fileName,
+        originalContent: result.originalContent ?? '',
+        newContent: result.newContent ?? content,
+        operation: 'write',
+      },
+    };
+
     return {
       success: true,
       output,
+      metadata,
     };
   },
 };