Adds a split button to the STL and 3MF preview toolbars that opens the
model directly in a desktop slicer via its URL scheme
(scheme://open?file=<absolute media URL>). The slicer downloads the
file itself over the network.
Offers Orca Slicer, Bambu Studio, and Cura; the last-used slicer is
remembered in localStorage and becomes the split button's default,
since browsers cannot detect which protocol handlers are installed.
PrusaSlicer is deliberately omitted: it only downloads from domains
whitelisted by Prusa (printables.com, thingiverse.com, cults3d.com),
so it cannot fetch from a WROLPi.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
