feat: add i18n language toggle and bilingual docs

webui/src/components/LanguageToggle.jsx

@@ -0,0 +1,18 @@
+import { useI18n } from '../i18n'
+
+export default function LanguageToggle({ className = '' }) {
+    const { lang, setLang, t } = useI18n()
+    const nextLang = lang === 'zh' ? 'en' : 'zh'
+    const label = nextLang === 'zh' ? t('language.chinese') : t('language.english')
+
+    return (
+        <button
+            type="button"
+            onClick={() => setLang(nextLang)}
+            className={`text-xs font-semibold px-2 py-1 rounded-md border border-border bg-secondary/50 text-muted-foreground hover:text-foreground hover:bg-secondary transition-colors ${className}`}
+            title={t('language.label')}
+        >
+            {label}
+        </button>
+    )
+}