/**
 * Formats a version string for display, handling null/undefined values.
 *
 * @param latestVersion - Latest version string or null/undefined.
 * @param currentVersion - Current version string or null/undefined.
 * @returns Formatted version string or 'unknown' placeholder.
 */
export declare function formatVersion(latestVersion: undefined | string | null, currentVersion: undefined | string | null): string;