Previewing a sliced multi-plate 3MF from the File Manager showed a plate
nobody picked. The library route took no plate parameter at all, so the
one the viewer has always put in the URL was dropped -- FastAPI discards
unknown query parameters silently. Both routes then fell back to the
first .gcode member of the zip, and member order is whatever the slicer
wrote: the reported file stores plate_2.gcode ahead of plate_1.gcode.
Nothing that opens the viewer from the File Manager passes a plate, so
there was no way to ask for another one either.
Plate resolution now lives in threemf_tools and both routes share it.
select_plate_gcode_name() returns the named plate or None, so a caller
serving an explicit choice can 404 instead of rendering something else;
default_plate_gcode_name() returns the lowest-numbered plate. The viewer
gained a plate switcher, and keeps the choice in its URL so a link to one
plate survives a reload. Filament colours follow it too -- they were
taken from the first plate regardless of which one was on screen.
G-code injection and the finish-photo max_z_height read shared the old
first-member fallback and now resolve the lowest plate as well.
