Add the mass and area conversion groups

[mairas]

Stacked on #571 (fix/server-unit-preference-vocabulary), which is where the preset-vocabulary spec table lives. Retarget to main once #571 merges.

Why

getConversionsForPath filters _conversionList for a group containing a measure equal to the path's SI unit. No group contained kg or m2, so the filter came back empty and the path degraded to unitless — even for the identity target, and on every preset including metric. design.displacement and a sail area rendered a raw number with no label.

This was split out of the #536 fix, which could not complete these two categories because the gap is not an alias: Skip had no conversion group for them at all.

What

category	SI base	targets
Mass	kg	kg, lbs
Area	m2	m2, sqft

Plus the matching unitConversionFunctions entries (js-quantities handles kg -> lbs and m^2 -> ft^2), and one alias — the server preset names its imperial mass target pound, following the same shape as the existing foot: 'feet'. sqft needs none; the server's target name and Skip's measure agree.

skBaseUnits already listed both kg (Mass) and m2 (Area) as valid Signal K units, so the type surface expected them; only the conversion table was missing.

The two categories move out of the "deliberately absent" notes in both SERVER_TARGET_UNIT_ALIASES and the spec's preset-vocabulary table, and into the table itself as four new rows.

src/assets/skip-dashboard-schema.json is the regenerated artifact (npm run gen:mcp-schema).

Tests

Three added, plus the four new vocabulary rows. All four cases fail without the groups (the existing preset-vocabulary test included).

Full suite: 2039 passing.

Fixes #570

[mairas]

Closed automatically by GitHub when the base branch fix/server-unit-preference-vocabulary was deleted on merging #571. A closed PR whose base is gone cannot be retargeted or reopened, so this continues as #583 — same branch and same commit, rebased onto main.