📦 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/core-js/internals/function-bind-context.js

@@ -0,0 +1,14 @@
+'use strict';
+var uncurryThis = require('../internals/function-uncurry-this-clause');
+var aCallable = require('../internals/a-callable');
+var NATIVE_BIND = require('../internals/function-bind-native');
+
+var bind = uncurryThis(uncurryThis.bind);
+
+// optional / simple context binding
+module.exports = function (fn, that) {
+  aCallable(fn);
+  return that === undefined ? fn : NATIVE_BIND ? bind(fn, that) : function (/* ...args */) {
+    return fn.apply(that, arguments);
+  };
+};