Add Microwaveprop.Format.distance_km/1 and a matching formatDistanceKm
helper for the JS side. Both emit "X mi (Y km)" with one decimal under
10 mi and whole numbers above.
Replace the ad-hoc format_dist/format_km_mi helpers with the shared
formatter and update every user-facing distance render: contact
detail, contacts index column, user profile contact/involving tables,
path finder summary + sounding list, contacts map line popups, beacon
coverage tooltip, and propagation map range estimate + rain scatter
cell popup.
