refactor: 将 WebUI 源码纳入仓库,移除打包产物

- 新增 webui/ 前端源码目录（React+Vite+TypeScript+Tailwind）
- 删除 api/webui/ 中旧的打包静态资源
- 更新 .gitignore 忽略 api/webui/ 构建输出和 webui/node_modules
- 更新 README 增加前端构建说明

webui/src/components/layout/LanguageSwitch.tsx

@@ -0,0 +1,36 @@
+import { Globe } from 'lucide-react'
+import { useTranslation } from 'react-i18next'
+import {
+  Select,
+  SelectContent,
+  SelectItem,
+  SelectTrigger,
+  SelectValue,
+} from '@/components/ui/select'
+
+const languages = [
+  { code: 'zh-CN', label: '中文' },
+  { code: 'en-US', label: 'EN' },
+]
+
+export function LanguageSwitch() {
+  const { i18n } = useTranslation()
+
+  const currentLang = languages.find(l => l.code === i18n.language) || languages[0]
+
+  return (
+    <Select value={i18n.language} onValueChange={(lang) => i18n.changeLanguage(lang)}>
+      <SelectTrigger className="w-20 h-7 text-xs font-mono border-cyber-border-subtle bg-cyber-bg-tertiary/50 hover:border-cyber-neon-cyan/50 transition-colors">
+        <Globe className="w-3 h-3 mr-1 text-cyber-text-secondary" />
+        <SelectValue>{currentLang.label}</SelectValue>
+      </SelectTrigger>
+      <SelectContent>
+        {languages.map((lang) => (
+          <SelectItem key={lang.code} value={lang.code} className="text-xs font-mono">
+            {lang.label}
+          </SelectItem>
+        ))}
+      </SelectContent>
+    </Select>
+  )
+}