📦 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.function.has-instance.js

@@ -0,0 +1,20 @@
+'use strict';
+var isCallable = require('../internals/is-callable');
+var isObject = require('../internals/is-object');
+var definePropertyModule = require('../internals/object-define-property');
+var isPrototypeOf = require('../internals/object-is-prototype-of');
+var wellKnownSymbol = require('../internals/well-known-symbol');
+var makeBuiltIn = require('../internals/make-built-in');
+
+var HAS_INSTANCE = wellKnownSymbol('hasInstance');
+var FunctionPrototype = Function.prototype;
+
+// `Function.prototype[@@hasInstance]` method
+// https://tc39.es/ecma262/#sec-function.prototype-@@hasinstance
+if (!(HAS_INSTANCE in FunctionPrototype)) {
+  definePropertyModule.f(FunctionPrototype, HAS_INSTANCE, { value: makeBuiltIn(function (O) {
+    if (!isCallable(this) || !isObject(O)) return false;
+    var P = this.prototype;
+    return isObject(P) ? isPrototypeOf(P, O) : O instanceof this;
+  }, HAS_INSTANCE) });
+}