📦 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-plugin-polyfill-corejs2/README.md

@@ -0,0 +1,28 @@
+# babel-plugin-polyfill-corejs2
+
+## Install
+
+Using npm:
+
+```sh
+npm install --save-dev babel-plugin-polyfill-corejs2
+```
+
+or using yarn:
+
+```sh
+yarn add babel-plugin-polyfill-corejs2 --dev
+```
+
+## Usage
+
+Add this plugin to your Babel configuration:
+
+```json
+{
+  "plugins": [["polyfill-corejs2", { "method": "usage-global" }]]
+}
+```
+
+This package supports the `usage-pure`, `usage-global`, and `entry-global` methods.
+When `entry-global` is used, it replaces imports to `core-js`.