feat(i18n): add multi-language support (ja/ko/vi/ru/zh-TW) with system auto-detection

- Extend AppLanguage to 7 locales: zh-CN, zh-TW, en, ja, ko, vi, ru
- Auto-detect system language via navigator.languages; fallback to English
- Add BCP47 locale mapping for date formatting and sorting
- Create 50 translation JSON files (5 languages × 10 namespaces, 1137 keys each)
- Update language selector with native language names
- Extend locale key parity test to cover all 7 languages (70 assertions)
- Add VNC/RDP desktop, serial terminal, AI agent, port forwarding, and SFTP features
- Add credential encryption, host metrics, session backup crypto, and zmodem bridge
- Update Rust backend: desktop services, host metrics, SSH/SFTP enhancements

src/components/common/DefaultFallback.tsx

@@ -1,3 +1,4 @@
+import { useTranslation } from 'react-i18next'
 import { handleError } from '../../utils/error-handler'
 
 interface DefaultFallbackProps {
@@ -6,6 +7,7 @@ interface DefaultFallbackProps {
 }
 
 export default function DefaultFallback({ error, onReset }: DefaultFallbackProps) {
+  const { t } = useTranslation('common')
   const appError = handleError(error)
 
   return (
@@ -18,15 +20,15 @@ export default function DefaultFallback({ error, onReset }: DefaultFallbackProps
             </svg>
           </div>
           <div>
-            <h2 className="text-lg font-semibold text-content-primary">出错了</h2>
-            <p className="text-sm text-txt-muted">应用遇到意外问题</p>
+            <h2 className="text-lg font-semibold text-content-primary">{t('error.title')}</h2>
+            <p className="text-sm text-txt-muted">{t('error.body')}</p>
           </div>
         </div>
 
         <div className="bg-dark-900 rounded-lg p-4 mb-4">
           <p className="text-sm text-content-secondary mb-2">{appError.message}</p>
           {appError.recoverable && (
-            <p className="text-xs text-green-400">此错误可以恢复，请尝试刷新页面</p>
+            <p className="text-xs text-green-400">{t('error.recoverable')}</p>
           )}
         </div>
 
@@ -35,18 +37,18 @@ export default function DefaultFallback({ error, onReset }: DefaultFallbackProps
             onClick={onReset}
             className="flex-1 px-4 py-2 bg-accent-blue hover:bg-blue-600 text-content-primary rounded-lg transition-colors text-sm font-medium"
           >
-            刷新页面
+            {t('error.refreshPage')}
           </button>
           <button
             onClick={() => window.location.reload()}
             className="px-4 py-2 bg-dark-700 hover:bg-dark-600 text-content-secondary rounded-lg transition-colors text-sm"
           >
-            重启应用
+            {t('error.restartApp')}
           </button>
         </div>
 
         <p className="mt-4 text-xs text-txt-faint text-center">
-          如果问题持续存在，请检查日志或联系支持
+          {t('error.supportHint')}
         </p>
       </div>
     </div>