/**
 * Convert a User-Agent string into a short, human-readable label. Meant
 * for a settings/sessions list — deliberately lossy so users see
 * "iPhone (iOS 17.4) · Safari" instead of parsing 300 characters of UA.
 *
 * @param {string | undefined | null} ua
 * @returns {string}                          Empty string when input is not a usable UA.
 */
export function deriveDeviceLabel(ua: string | undefined | null): string;