📦 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/detect-node/Readme.md

@@ -0,0 +1,30 @@
+### Install
+
+```shell
+npm install --save detect-node
+```
+
+### Usage:
+
+```js
+var isNode = require('detect-node');
+
+if (isNode) {
+  console.log("Running under Node.JS");
+} else {
+  alert("Hello from browser (or whatever not-a-node env)");
+}
+```
+
+The check is performed as:
+```js
+module.exports = false;
+
+// Only Node.JS has a process variable that is of [[Class]] process
+try {
+ module.exports = Object.prototype.toString.call(global.process) === '[object process]' 
+} catch(e) {}
+
+```
+
+Thanks to Ingvar Stepanyan for the initial idea. This check is both **the most reliable I could find** and it does not use `process` env directly, which would cause browserify to include it into the build.