The nozzle_info MQTT array contains L/R nozzle heads (IDs 0, 1) and
rack slots (IDs 16-21). Backend was dumping all entries into nozzle_rack,
and frontend slice(0,6) grabbed L, R, plus only 4 rack slots — cutting
off the last position (e.g. the 0.6mm nozzle) and showing the mounted
nozzle as docked.
Backend: filter nozzle_rack to id >= 2 (rack-only), sort by ID.
Frontend: compact single-row layout with bottom accent bars for
mounted/docked status, wider rack card (flex-[2]), vertically
centered temp cards.