📦 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-strategies/plugins/cacheOkAndOpaquePlugin.d.ts

@@ -0,0 +1,3 @@
+import { WorkboxPlugin } from 'workbox-core/types.js';
+import '../_version.js';
+export declare const cacheOkAndOpaquePlugin: WorkboxPlugin;

frontend/node_modules/workbox-strategies/plugins/cacheOkAndOpaquePlugin.js

@@ -0,0 +1,26 @@
+/*
+  Copyright 2018 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';
+export const cacheOkAndOpaquePlugin = {
+    /**
+     * Returns a valid response (to allow caching) if the status is 200 (OK) or
+     * 0 (opaque).
+     *
+     * @param {Object} options
+     * @param {Response} options.response
+     * @return {Response|null}
+     *
+     * @private
+     */
+    cacheWillUpdate: async ({ response }) => {
+        if (response.status === 200 || response.status === 0) {
+            return response;
+        }
+        return null;
+    },
+};

frontend/node_modules/workbox-strategies/plugins/cacheOkAndOpaquePlugin.mjs

@@ -0,0 +1 @@
+export * from './cacheOkAndOpaquePlugin.js';