📦 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/encodeurl/index.js

@@ -0,0 +1,60 @@
+/*!
+ * encodeurl
+ * Copyright(c) 2016 Douglas Christopher Wilson
+ * MIT Licensed
+ */
+
+'use strict'
+
+/**
+ * Module exports.
+ * @public
+ */
+
+module.exports = encodeUrl
+
+/**
+ * RegExp to match non-URL code points, *after* encoding (i.e. not including "%")
+ * and including invalid escape sequences.
+ * @private
+ */
+
+var ENCODE_CHARS_REGEXP = /(?:[^\x21\x23-\x3B\x3D\x3F-\x5F\x61-\x7A\x7C\x7E]|%(?:[^0-9A-Fa-f]|[0-9A-Fa-f][^0-9A-Fa-f]|$))+/g
+
+/**
+ * RegExp to match unmatched surrogate pair.
+ * @private
+ */
+
+var UNMATCHED_SURROGATE_PAIR_REGEXP = /(^|[^\uD800-\uDBFF])[\uDC00-\uDFFF]|[\uD800-\uDBFF]([^\uDC00-\uDFFF]|$)/g
+
+/**
+ * String to replace unmatched surrogate pair with.
+ * @private
+ */
+
+var UNMATCHED_SURROGATE_PAIR_REPLACE = '$1\uFFFD$2'
+
+/**
+ * Encode a URL to a percent-encoded form, excluding already-encoded sequences.
+ *
+ * This function will take an already-encoded URL and encode all the non-URL
+ * code points. This function will not encode the "%" character unless it is
+ * not part of a valid sequence (`%20` will be left as-is, but `%foo` will
+ * be encoded as `%25foo`).
+ *
+ * This encode is meant to be "safe" and does not throw errors. It will try as
+ * hard as it can to properly encode the given URL, including replacing any raw,
+ * unpaired surrogate pairs with the Unicode replacement character prior to
+ * encoding.
+ *
+ * @param {string} url
+ * @return {string}
+ * @public
+ */
+
+function encodeUrl (url) {
+  return String(url)
+    .replace(UNMATCHED_SURROGATE_PAIR_REGEXP, UNMATCHED_SURROGATE_PAIR_REPLACE)
+    .replace(ENCODE_CHARS_REGEXP, encodeURI)
+}