Body composition tracking (lean mass, muscle, bone density)

Extend tracking beyond weight and body fat % to full body composition: lean muscle mass, skeletal muscle mass, body fat mass, bone density, with room for additional metrics later (visceral fat, body water, etc.). Manual entry first so smart-scale and DEXA users can log values from outside sources; phase 2 could auto-sync from Apple Health / Health Connect, which already expose lean body mass and bone metrics — many scale apps (Hume, Withings, Renpho) write there.