📦 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.date.to-primitive.js

@@ -0,0 +1,14 @@
+'use strict';
+var hasOwn = require('../internals/has-own-property');
+var defineBuiltIn = require('../internals/define-built-in');
+var dateToPrimitive = require('../internals/date-to-primitive');
+var wellKnownSymbol = require('../internals/well-known-symbol');
+
+var TO_PRIMITIVE = wellKnownSymbol('toPrimitive');
+var DatePrototype = Date.prototype;
+
+// `Date.prototype[@@toPrimitive]` method
+// https://tc39.es/ecma262/#sec-date.prototype-@@toprimitive
+if (!hasOwn(DatePrototype, TO_PRIMITIVE)) {
+  defineBuiltIn(DatePrototype, TO_PRIMITIVE, dateToPrimitive);
+}