📦 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/modules/es.array.join.js

@@ -0,0 +1,19 @@
+'use strict';
+var $ = require('../internals/export');
+var uncurryThis = require('../internals/function-uncurry-this');
+var IndexedObject = require('../internals/indexed-object');
+var toIndexedObject = require('../internals/to-indexed-object');
+var arrayMethodIsStrict = require('../internals/array-method-is-strict');
+
+var nativeJoin = uncurryThis([].join);
+
+var ES3_STRINGS = IndexedObject !== Object;
+var FORCED = ES3_STRINGS || !arrayMethodIsStrict('join', ',');
+
+// `Array.prototype.join` method
+// https://tc39.es/ecma262/#sec-array.prototype.join
+$({ target: 'Array', proto: true, forced: FORCED }, {
+  join: function join(separator) {
+    return nativeJoin(toIndexedObject(this), separator === undefined ? ',' : separator);
+  }
+});