Add "Open in OrcaSlicer" — one-click slicer hand-off for generated models #346

Modly generates meshes for 3D printing, but getting one into a slicer today means manually exporting an STL and importing it by hand.

Proposal: an Open in OrcaSlicer item in the Export dropdown that converts the current model to STL and hands it to OrcaSlicer via its orcaslicer://open?file=<url> deeplink, served from the local backend.

Why a deeplink + STL: OrcaSlicer registers orcaslicer://open?file=<url>, downloads the URL, and derives the import format from the URL's final path segment — and it cannot import GLB. So the model is converted to STL on the fly and served at a path-only URL ending in model.stl.

Scope (v1):

• STL only (universal; OrcaSlicer auto-repairs). 3MF is a possible fast-follow.
• Reorient Y-up → Z-up and normalize to a sane print size so it imports upright and visible.
• Graceful fallback when OrcaSlicer is not installed.

Happy to implement.