📦 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/babel-preset-react-app/index.js

@@ -0,0 +1,20 @@
+/**
+ * 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 create = require('./create');
+
+module.exports = function (api, opts) {
+  // This is similar to how `env` works in Babel:
+  // https://babeljs.io/docs/usage/babelrc/#env-option
+  // We are not using `env` because it’s ignored in versions > babel-core@6.10.4:
+  // https://github.com/babel/babel/issues/4539
+  // https://github.com/facebook/create-react-app/issues/720
+  // It’s also nice that we can enforce `NODE_ENV` being specified.
+  const env = process.env.BABEL_ENV || process.env.NODE_ENV;
+  return create(api, opts, env);
+};