📦 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-pure/modules/es.array.of.js

@@ -0,0 +1,28 @@
+'use strict';
+var $ = require('../internals/export');
+var fails = require('../internals/fails');
+var isConstructor = require('../internals/is-constructor');
+var createProperty = require('../internals/create-property');
+var setArrayLength = require('../internals/array-set-length');
+
+var $Array = Array;
+
+var ISNT_GENERIC = fails(function () {
+  function F() { /* empty */ }
+  // eslint-disable-next-line es/no-array-of -- safe
+  return !($Array.of.call(F) instanceof F);
+});
+
+// `Array.of` method
+// https://tc39.es/ecma262/#sec-array.of
+// WebKit Array.of isn't generic
+$({ target: 'Array', stat: true, forced: ISNT_GENERIC }, {
+  of: function of(/* ...args */) {
+    var index = 0;
+    var argumentsLength = arguments.length;
+    var result = new (isConstructor(this) ? this : $Array)(argumentsLength);
+    while (argumentsLength > index) createProperty(result, index, arguments[index++]);
+    setArrayLength(result, argumentsLength);
+    return result;
+  }
+});