feat(auth): show token expiry in days/weeks instead of raw hours

formatDuration() now supports 4 tiers: minutes, hours+minutes,
days+hours, and weeks+days. Previously it maxed out at hours, producing
awkward output like "580 hours and 16 minutes remaining" for tokens
that expire in ~24 days.

Extracted pluralUnit() and durationPair() helpers to keep the function
under Biome complexity limit of 15.

Author: BYK