📦 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/performance-now/src/index.d.ts

@@ -0,0 +1,8 @@
+// This file describes the package to typescript.
+
+/**
+ * Returns the number of milliseconds since the page was loaded (if browser)
+ * or the node process was started.
+ */
+declare function now(): number;
+export = now;

frontend/node_modules/performance-now/src/performance-now.coffee

@@ -0,0 +1,17 @@
+if performance? and performance.now
+  module.exports = -> performance.now()
+else if process? and process.hrtime
+  module.exports = -> (getNanoSeconds() - nodeLoadTime) / 1e6
+  hrtime = process.hrtime
+  getNanoSeconds = ->
+    hr = hrtime()
+    hr[0] * 1e9 + hr[1]
+  moduleLoadTime = getNanoSeconds()
+  upTime = process.uptime() * 1e9
+  nodeLoadTime = moduleLoadTime - upTime
+else if Date.now
+  module.exports = -> Date.now() - loadTime
+  loadTime = Date.now()
+else
+  module.exports = -> new Date().getTime() - loadTime
+  loadTime = new Date().getTime()