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).

README.md

@@ -130,7 +130,7 @@ rebuild with no errors, replayed here at 2× speed.
   IGES 5.3 (manifold solid B-reps with names and colours), 3MF, STL (binary/ASCII) and OBJ, and
   assemblies with joints as URDF for robot simulators (docs/urdf.md);
   3MF, STL and OBJ import as mesh bodies (they render, measure, move and export; solid features
-  need B-rep bodies). Convert Mesh reconstructs supported closed constant-section parts
+  need B-rep bodies). The `mesh.convert` command reconstructs supported closed constant-section parts
   as parametric solids with planar and circular cylindrical faces;
   STEP import (AP203/AP214/AP242 solids, assemblies, units, names, colours) and IGES import
   (manifold solid B-reps, or trimmed and bounded surfaces sewn into solids) as a base feature