📦 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-app-polyfill/package.json

@@ -0,0 +1,32 @@
+{
+  "name": "react-app-polyfill",
+  "version": "3.0.0",
+  "description": "Polyfills for various browsers including commonly used language features",
+  "repository": {
+    "type": "git",
+    "url": "https://github.com/facebook/create-react-app.git",
+    "directory": "packages/react-app-polyfill"
+  },
+  "license": "MIT",
+  "bugs": {
+    "url": "https://github.com/facebook/create-react-app/issues"
+  },
+  "engines": {
+    "node": ">=14"
+  },
+  "files": [
+    "ie9.js",
+    "ie11.js",
+    "jsdom.js",
+    "stable.js"
+  ],
+  "dependencies": {
+    "core-js": "^3.19.2",
+    "object-assign": "^4.1.1",
+    "promise": "^8.1.0",
+    "raf": "^3.4.1",
+    "regenerator-runtime": "^0.13.9",
+    "whatwg-fetch": "^3.6.2"
+  },
+  "gitHead": "221e511730ca51c036c6954a9d2ee7659ff860f9"
+}