📦 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/workbox-core/clientsClaim.js

@@ -0,0 +1,18 @@
+/*
+  Copyright 2019 Google LLC
+
+  Use of this source code is governed by an MIT-style
+  license that can be found in the LICENSE file or at
+  https://opensource.org/licenses/MIT.
+*/
+import './_version.js';
+/**
+ * Claim any currently available clients once the service worker
+ * becomes active. This is normally used in conjunction with `skipWaiting()`.
+ *
+ * @memberof workbox-core
+ */
+function clientsClaim() {
+    self.addEventListener('activate', () => self.clients.claim());
+}
+export { clientsClaim };