📦 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-router/README.md

@@ -0,0 +1,16 @@
+# React Router
+
+The `react-router` package is the heart of [React Router](https://github.com/remix-run/react-router) and provides all
+the core functionality for both
+[`react-router-dom`](https://github.com/remix-run/react-router/tree/main/packages/react-router-dom)
+and
+[`react-router-native`](https://github.com/remix-run/react-router/tree/main/packages/react-router-native).
+
+If you're using React Router, you should never `import` anything directly from
+the `react-router` package, but you should have everything you need in either
+`react-router-dom` or `react-router-native`. Both of those packages re-export
+everything from `react-router`.
+
+If you'd like to extend React Router and you know what you're doing, you should
+add `react-router` **as a peer dependency, not a regular dependency** in your
+package.