The metric/imperial preference was already honored by every length input,
but two area surfaces were still hardcoded to m²:
- the "Load build" import dialog's Floor area stat
- the auto-generated default names for zones, slabs, and ceilings
(e.g. "Zone (12.3m²)"), which are live display fallbacks
Both now convert value + label to the active unit. Adds shared
squareMetersToAreaUnit / getAreaUnitLabel / formatAreaLabel helpers to
lib/measurements (with tests) and routes the site-panel Area readout
through them, replacing an inline 10.7639 magic constant.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
