/**
 * Format a duration between two dates as a human-readable string.
 */
export function formatDuration(start: Date, end?: Date): string {
  const duration = (end ?? new Date()).getTime() - start.getTime()
  const seconds = Math.floor(duration / 1000)
  const minutes = Math.floor(seconds / 60)
  const hours = Math.floor(minutes / 60)

  if (hours > 0) return `${hours}h ${minutes % 60}m ${seconds % 60}s`
  if (minutes > 0) return `${minutes}m ${seconds % 60}s`
  return `${seconds}s`
}