v0.13.1

Fixed

• Fixed date arithmetic for months and years to use proper calendar semantics instead of
  fixed-second approximations (e.g. 17 February 2027 - 6 months now correctly returns
  2026-08-17 instead of 2026-08-21). The day-of-month is now preserved, with clamping
  to the last day of the month when needed (e.g. 31 January + 1 month = 28 February).
• Fixed duration unit display to use full English words (months, years, days, etc.)
  instead of abbreviations (mo, y, d, etc.) in calculation steps and lino notation.