📦 添加虚拟环境和启动脚本

新增:
- backend/venv/ - Python 虚拟环境
- backend/start.sh - 启动脚本（使用虚拟环境）
- backend/requirements.txt - 依赖列表
- .gitignore - 忽略虚拟环境和缓存文件

说明:
- 每个项目使用独立虚拟环境
- 避免依赖冲突
- 启动脚本自动创建和激活虚拟环境

frontend/node_modules/react-dev-utils/errorOverlayMiddleware.js

@@ -0,0 +1,23 @@
+/**
+ * Copyright (c) 2015-present, Facebook, Inc.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+'use strict';
+
+const launchEditor = require('./launchEditor');
+const launchEditorEndpoint = require('./launchEditorEndpoint');
+
+module.exports = function createLaunchEditorMiddleware() {
+  return function launchEditorMiddleware(req, res, next) {
+    if (req.url.startsWith(launchEditorEndpoint)) {
+      const lineNumber = parseInt(req.query.lineNumber, 10) || 1;
+      const colNumber = parseInt(req.query.colNumber, 10) || 1;
+      launchEditor(req.query.fileName, lineNumber, colNumber);
+      res.end();
+    } else {
+      next();
+    }
+  };
+};