📦 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/axios/lib/helpers/isAbsoluteURL.js

@@ -0,0 +1,19 @@
+'use strict';
+
+/**
+ * Determines whether the specified URL is absolute
+ *
+ * @param {string} url The URL to test
+ *
+ * @returns {boolean} True if the specified URL is absolute, otherwise false
+ */
+export default function isAbsoluteURL(url) {
+  // A URL is considered absolute if it begins with "<scheme>://" or "//" (protocol-relative URL).
+  // RFC 3986 defines scheme name as a sequence of characters beginning with a letter and followed
+  // by any combination of letters, digits, plus, period, or hyphen.
+  if (typeof url !== 'string') {
+    return false;
+  }
+
+  return /^([a-z][a-z\d+\-.]*:)?\/\//i.test(url);
+}