Save as mesh: one dialog for STL and 3MF, with Send to slicer (#1272)

One docked Save as mesh dialog replaces Export STL / 3MF / 3MF with face tags, with an optional Send to slicer that lists the slicers installed on this machine. Engine: file.save_mesh and file.slicers; the Convert Mesh dialog goes (mesh.convert and the MeshConvert feature stay, so existing designs open). Strings in all catalogs; screenshots in the PR. Checked on main + this branch: cargo check --workspace, the engine slicer/mesh/file tests, the UI catalog and menu tests and scenario s14 (the one local colour-token failure is epaint's TexturesDelta panic, which main shows on this machine too).

docs/formats/3mf-face-tags.md

@@ -7,7 +7,7 @@ A tessellated body loses its faces: a plain mesh says where the surface is, not
 each triangle came from. Downstream tools that mesh or analyse a part need that back. They apply
 loads and boundary conditions to faces, keep face boundaries as feature edges, and group
 results by face. SolidCraft therefore exports a 3MF in which every triangle carries the index of
-the B-rep face it lies on (**File → Export 3MF with Face Tags**, or `file.export` with
+the B-rep face it lies on (**File → Save as mesh…** (format **3MF with face tags**), or `file.export` with
 `format: "3mf-tagged"`).
 
 ## Why 3MF