Add colored OBJ import workflow

[davidkrammer]

Adds colored OBJ support to Primed3D.

Changes:

• OBJ uploads can now include MTL and texture images.
• After loading an OBJ, the UI shows OBJ-only buttons to add or replace the MTL and texture image without reselecting the OBJ.
• Imported OBJ texture/material/vertex colors are converted into Primed3D face colors, so the existing processing and 3MF export flow can use them.
• The export info shown after processing now includes the model size in mm, so scale mistakes are easier to catch before downloading.

Verified:

• JavaScript syntax check passed.
• git diff --check passed.
• Browser-tested OBJ alone, OBJ plus MTL, and OBJ plus texture image.
• Browser-tested both Solid and CMY processing summaries showing model size.