📦 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/regexp-exec-abstract.js

@@ -0,0 +1,21 @@
+'use strict';
+var call = require('../internals/function-call');
+var anObject = require('../internals/an-object');
+var isCallable = require('../internals/is-callable');
+var classof = require('../internals/classof-raw');
+var regexpExec = require('../internals/regexp-exec');
+
+var $TypeError = TypeError;
+
+// `RegExpExec` abstract operation
+// https://tc39.es/ecma262/#sec-regexpexec
+module.exports = function (R, S) {
+  var exec = R.exec;
+  if (isCallable(exec)) {
+    var result = call(exec, R, S);
+    if (result !== null) anObject(result);
+    return result;
+  }
+  if (classof(R) === 'RegExp') return call(regexpExec, R, S);
+  throw new $TypeError('RegExp#exec called on incompatible receiver');
+};