📦 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/2015/StringGetIndexProperty.js

@@ -0,0 +1,50 @@
+'use strict';
+
+var callBound = require('call-bound');
+
+var $TypeError = require('es-errors/type');
+
+var $charAt = callBound('String.prototype.charAt');
+
+var isString = require('is-string');
+var isNegativeZero = require('math-intrinsics/isNegativeZero');
+var unbox = require('unbox-primitive');
+
+var CanonicalNumericIndexString = require('./CanonicalNumericIndexString');
+var IsInteger = require('./IsInteger');
+var isPropertyKey = require('../helpers/isPropertyKey');
+
+// https://262.ecma-international.org/6.0/#sec-stringgetindexproperty
+
+module.exports = function StringGetIndexProperty(S, P) {
+	if (typeof S === 'string' || !isString(S)) {
+		throw new $TypeError('Assertion failed: `S` must be a boxed String Object');
+	}
+	if (!isPropertyKey(P)) {
+		throw new $TypeError('Assertion failed: `P` must be a Property Key');
+	}
+
+	if (typeof P !== 'string') {
+		return void undefined;
+	}
+
+	var index = CanonicalNumericIndexString(P);
+	if (typeof index === 'undefined' || !IsInteger(index) || isNegativeZero(index)) {
+		return void undefined;
+	}
+
+	var str = unbox(S);
+	var len = str.length;
+	if (index < 0 || len <= index) {
+		return void undefined;
+	}
+
+	var resultStr = $charAt(str, index);
+
+	return {
+		'[[Configurable]]': false,
+		'[[Enumerable]]': true,
+		'[[Value]]': resultStr,
+		'[[Writable]]': false
+	};
+};