📦 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/isexe/mode.js

@@ -0,0 +1,41 @@
+module.exports = isexe
+isexe.sync = sync
+
+var fs = require('fs')
+
+function isexe (path, options, cb) {
+  fs.stat(path, function (er, stat) {
+    cb(er, er ? false : checkStat(stat, options))
+  })
+}
+
+function sync (path, options) {
+  return checkStat(fs.statSync(path), options)
+}
+
+function checkStat (stat, options) {
+  return stat.isFile() && checkMode(stat, options)
+}
+
+function checkMode (stat, options) {
+  var mod = stat.mode
+  var uid = stat.uid
+  var gid = stat.gid
+
+  var myUid = options.uid !== undefined ?
+    options.uid : process.getuid && process.getuid()
+  var myGid = options.gid !== undefined ?
+    options.gid : process.getgid && process.getgid()
+
+  var u = parseInt('100', 8)
+  var g = parseInt('010', 8)
+  var o = parseInt('001', 8)
+  var ug = u | g
+
+  var ret = (mod & o) ||
+    (mod & g) && gid === myGid ||
+    (mod & u) && uid === myUid ||
+    (mod & ug) && myUid === 0
+
+  return ret
+}