📦 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/es-abstract/2023/substring.js

@@ -0,0 +1,15 @@
+'use strict';
+
+var $TypeError = require('es-errors/type');
+var isInteger = require('math-intrinsics/isInteger');
+var callBound = require('call-bound');
+
+var $slice = callBound('String.prototype.slice');
+
+// https://262.ecma-international.org/12.0/#substring
+module.exports = function substring(S, inclusiveStart, exclusiveEnd) {
+	if (typeof S !== 'string' || !isInteger(inclusiveStart) || (arguments.length > 2 && !isInteger(exclusiveEnd))) {
+		throw new $TypeError('`S` must be a String, and `inclusiveStart` and `exclusiveEnd` must be integers');
+	}
+	return $slice(S, inclusiveStart, arguments.length > 2 ? exclusiveEnd : S.length);
+};