📦 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/yargs-parser/browser.js

@@ -0,0 +1,29 @@
+// Main entrypoint for ESM web browser environments. Avoids using Node.js
+// specific libraries, such as "path".
+//
+// TODO: figure out reasonable web equivalents for "resolve", "normalize", etc.
+import { camelCase, decamelize, looksLikeNumber } from './build/lib/string-utils.js'
+import { YargsParser } from './build/lib/yargs-parser.js'
+const parser = new YargsParser({
+  cwd: () => { return '' },
+  format: (str, arg) => { return str.replace('%s', arg) },
+  normalize: (str) => { return str },
+  resolve: (str) => { return str },
+  require: () => {
+    throw Error('loading config from files not currently supported in browser')
+  },
+  env: () => {}
+})
+
+const yargsParser = function Parser (args, opts) {
+  const result = parser.parse(args.slice(), opts)
+  return result.argv
+}
+yargsParser.detailed = function (args, opts) {
+  return parser.parse(args.slice(), opts)
+}
+yargsParser.camelCase = camelCase
+yargsParser.decamelize = decamelize
+yargsParser.looksLikeNumber = looksLikeNumber
+
+export default yargsParser