📦 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/@ungap/structured-clone/esm/index.js

@@ -0,0 +1,25 @@
+import {deserialize} from './deserialize.js';
+import {serialize} from './serialize.js';
+
+/**
+ * @typedef {Array<string,any>} Record a type representation
+ */
+
+/**
+ * Returns an array of serialized Records.
+ * @param {any} any a serializable value.
+ * @param {{transfer?: any[], json?: boolean, lossy?: boolean}?} options an object with
+ * a transfer option (ignored when polyfilled) and/or non standard fields that
+ * fallback to the polyfill if present.
+ * @returns {Record[]}
+ */
+export default typeof structuredClone === "function" ?
+  /* c8 ignore start */
+  (any, options) => (
+    options && ('json' in options || 'lossy' in options) ?
+      deserialize(serialize(any, options)) : structuredClone(any)
+  ) :
+  (any, options) => deserialize(serialize(any, options));
+  /* c8 ignore stop */
+
+export {deserialize, serialize};