The PO line-items Unit of Measure dropdown only offered hr/day among
time-based units. Add week, month and year so durations beyond days can
be selected, as requested. UOM_OPTIONS is the single source of truth and
`unit` is validated as a free-form string, so no schema/validation change
is needed.
Fixes#44
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>