📦 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/globalthis/shim.js

@@ -0,0 +1,29 @@
+'use strict';
+
+var define = require('define-properties');
+var gOPD = require('gopd');
+var getPolyfill = require('./polyfill');
+
+module.exports = function shimGlobal() {
+	var polyfill = getPolyfill();
+	if (define.supportsDescriptors) {
+		var descriptor = gOPD(polyfill, 'globalThis');
+		if (
+			!descriptor
+			|| (
+				descriptor.configurable
+				&& (descriptor.enumerable || !descriptor.writable || globalThis !== polyfill)
+			)
+		) {
+			Object.defineProperty(polyfill, 'globalThis', {
+				configurable: true,
+				enumerable: false,
+				value: polyfill,
+				writable: true
+			});
+		}
+	} else if (typeof globalThis !== 'object' || globalThis !== polyfill) {
+		polyfill.globalThis = polyfill;
+	}
+	return polyfill;
+};