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).

crates/engine/src/cmd/mesh.rs

@@ -15,7 +15,6 @@ use crate::{
 
 pub static COMMANDS: &[CommandSpec] = &[
     CommandSpec::new("mesh.convert", "Convert Mesh", convert)
-        .at("MESH", "MODIFY")
         .icon("import")
         .params("body: mesh body name; tolerance?: length expression (default 0.05 mm): reconstruct a closed constant-section mechanical mesh as editable planar and cylindrical faces, with a parametric timeline feature; name?"),
 ];