Format a date as a relative time string (e.g., "Today", "Yesterday", "3d ago").
Uses calendar-day boundaries in the user's local timezone, so a timestamp
from 11pm last night reads "Yesterday" at 1am rather than "Today".
Parameters
date: string|Date|null
Date to format (Date object, date string, or null)
Returns string
Human-readable relative time string, or "Never" for null/invalid dates
Format a date as a relative time string (e.g., "Today", "Yesterday", "3d ago"). Uses calendar-day boundaries in the user's local timezone, so a timestamp from 11pm last night reads "Yesterday" at 1am rather than "Today".