dateUtils: Add formatRelative function for human-readable relative dates

[Kingsman-99]

Description

dateUtils provides absolute date formatting but no relative formatting. Contributors should add a formatRelative(date: Date, now?: Date): string function that returns human-readable strings like "just now", "2 minutes ago", "3 days ago", and "in 5 hours", capped at "over a year ago" / "over a year from now" for distant dates.

Acceptance Criteria

• formatRelative returns "just now" for dates within 60 seconds of now
• It returns minute, hour, day, week, and month variants as appropriate
• It accepts an optional now parameter (defaults to new Date()) for testability
• Existing tests continue to pass

Context

• Target file: src/lib/dateUtils.ts
• Do not import any third-party date library — implement purely with native Date arithmetic