📦 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/indexed-object.js

@@ -0,0 +1,16 @@
+'use strict';
+var uncurryThis = require('../internals/function-uncurry-this');
+var fails = require('../internals/fails');
+var classof = require('../internals/classof-raw');
+
+var $Object = Object;
+var split = uncurryThis(''.split);
+
+// fallback for non-array-like ES3 and non-enumerable old V8 strings
+module.exports = fails(function () {
+  // throws an error in rhino, see https://github.com/mozilla/rhino/issues/346
+  // eslint-disable-next-line no-prototype-builtins -- safe
+  return !$Object('z').propertyIsEnumerable(0);
+}) ? function (it) {
+  return classof(it) === 'String' ? split(it, '') : $Object(it);
+} : $Object;