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/ui/package.json

@@ -28,6 +28,8 @@
     "@codemirror/lang-json": "^6.0.2",
     "@codemirror/lang-markdown": "^6.5.0",
     "@codemirror/lang-python": "^6.2.1",
+    "@codemirror/merge": "^6.10.2",
+    "@codemirror/state": "^6.5.2",
     "@codemirror/theme-one-dark": "^6.1.3",
     "@radix-ui/react-dialog": "^1.1.15",
     "@radix-ui/react-dropdown-menu": "^2.1.16",
@@ -39,6 +41,7 @@
     "@uiw/react-codemirror": "^4.25.4",
     "class-variance-authority": "^0.7.1",
     "clsx": "^2.1.1",
+    "codemirror": "^6.0.2",
     "framer-motion": "^12.23.26",
     "lucide-react": "^0.344.0",
     "react-markdown": "^10.1.0",