📦 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/esnext.array.last-index.js

@@ -0,0 +1,22 @@
+'use strict';
+// TODO: Remove from `core-js@4`
+var DESCRIPTORS = require('../internals/descriptors');
+var addToUnscopables = require('../internals/add-to-unscopables');
+var toObject = require('../internals/to-object');
+var lengthOfArrayLike = require('../internals/length-of-array-like');
+var defineBuiltInAccessor = require('../internals/define-built-in-accessor');
+
+// `Array.prototype.lastIndex` getter
+// https://github.com/tc39/proposal-array-last
+if (DESCRIPTORS) {
+  defineBuiltInAccessor(Array.prototype, 'lastIndex', {
+    configurable: true,
+    get: function lastIndex() {
+      var O = toObject(this);
+      var len = lengthOfArrayLike(O);
+      return len === 0 ? 0 : len - 1;
+    }
+  });
+
+  addToUnscopables('lastIndex');
+}