📦 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-dom/server.node.js

@@ -0,0 +1,17 @@
+'use strict';
+
+var l, s;
+if (process.env.NODE_ENV === 'production') {
+  l = require('./cjs/react-dom-server-legacy.node.production.min.js');
+  s = require('./cjs/react-dom-server.node.production.min.js');
+} else {
+  l = require('./cjs/react-dom-server-legacy.node.development.js');
+  s = require('./cjs/react-dom-server.node.development.js');
+}
+
+exports.version = l.version;
+exports.renderToString = l.renderToString;
+exports.renderToStaticMarkup = l.renderToStaticMarkup;
+exports.renderToNodeStream = l.renderToNodeStream;
+exports.renderToStaticNodeStream = l.renderToStaticNodeStream;
+exports.renderToPipeableStream = s.renderToPipeableStream;