📦 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/2025/StringPaddingBuiltinsImpl.js

@@ -0,0 +1,27 @@
+'use strict';
+
+var $TypeError = require('es-errors/type');
+
+var StringPad = require('./StringPad');
+var ToLength = require('./ToLength');
+var ToString = require('./ToString');
+
+// https://262.ecma-international.org/15.0/#sec-stringpaddingbuiltinsimpl
+
+module.exports = function StringPaddingBuiltinsImpl(O, maxLength, fillString, placement) {
+	if (placement !== 'start' && placement !== 'end' && placement !== 'START' && placement !== 'END') {
+		throw new $TypeError('Assertion failed: `placement` must be ~START~ or ~END~');
+	}
+
+	var S = ToString(O); // step 1
+
+	var intMaxLength = ToLength(maxLength); // step 2
+
+	var stringLength = S.length; // step 3
+
+	if (intMaxLength <= stringLength) { return S; } // step 4
+
+	var filler = typeof fillString === 'undefined' ? ' ' : ToString(fillString); // steps 5-6
+
+	return StringPad(S, intMaxLength, filler, placement); // step 7
+};