Three fixes for the H2C nozzle rack reported by testers:
1. Empty slot position: rack nozzles were mapped by sequential array
index instead of by actual nozzle ID (16-21), so a missing slot
always appeared at position 6 instead of its real position. Now
maps each nozzle to its correct rack position via id offset.
2. Missing filament colors/materials: the H2C MQTT payload uses
different field names than H2D — color_m (not filament_colour),
fila_id (not filament_id), sn (not serial_number), tm (not
max_temp). Added fallback field name resolution in the backend
MQTT parser.
3. Medium card layout: the temperature row with nozzle rack overflowed
on medium-sized cards. Added flex-wrap and a 190px flex-basis on
the rack card so it wraps to a second line when space is tight.