fix(mcp): improve error message internationalization

- Add translateMcpBackendError utility to map backend errors to i18n keys
- Update error handling in McpPanel, McpFormModal, and McpWizardModal
- Internationalize stdio/http type selectors in Wizard
- Implement three-tier fallback strategy: translation → raw error → default message
- No backend changes required, fully frontend-based i18n implementation

src/components/mcp/McpWizardModal.tsx

@@ -227,7 +227,7 @@ const McpWizardModal: React.FC<McpWizardModalProps> = ({
                     className="w-4 h-4 text-emerald-500 bg-white dark:bg-gray-800 border-gray-200 dark:border-gray-700 focus:ring-emerald-500 dark:focus:ring-emerald-400 focus:ring-2"
                   />
                   <span className="text-sm text-gray-900 dark:text-gray-100">
-                    stdio
+                    {t("mcp.wizard.typeStdio")}
                   </span>
                 </label>
                 <label className="inline-flex items-center gap-2 cursor-pointer">
@@ -241,7 +241,7 @@ const McpWizardModal: React.FC<McpWizardModalProps> = ({
                     className="w-4 h-4 text-emerald-500 bg-white dark:bg-gray-800 border-gray-200 dark:border-gray-700 focus:ring-emerald-500 dark:focus:ring-emerald-400 focus:ring-2"
                   />
                   <span className="text-sm text-gray-900 dark:text-gray-100">
-                    http
+                    {t("mcp.wizard.typeHttp")}
                   </span>
                 </label>
               </div>