/**
 * Formats large numbers into compact human-readable strings (e.g. 1.2K, 3.4M, 5.1B).
 *
 * @example
 * formatCompactNumber(1200) // "1.2K"
 * formatCompactNumber(1500000) // "1.5M"
 *
 * @param num The number to format.
 * @param locale The locale to format with (default "en-US").
 * @returns Compact formatted number string.
 */
export declare function formatCompactNumber(num: number, locale?: string): string;