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).

apps/solidcraft/src/main.rs

@@ -98,6 +98,7 @@ impl eframe::App for App {
 fn services(cc: &eframe::CreationContext<'_>) -> Services {
     let parent = rfd::AsyncFileDialog::new().set_parent(cc);
     let open_parent = parent.clone();
+    let program_parent = parent.clone();
     Services {
         pick_open: Some(Box::new(move || {
             let future = open_parent
@@ -131,6 +132,12 @@ fn services(cc: &eframe::CreationContext<'_>) -> Services {
                 .pick_file();
             Box::pin(async move { future.await.map(|p| p.path().to_string_lossy().into_owned()) })
         })),
+        pick_program: Some(Box::new(move || {
+            let dialog = program_parent.clone().set_title("Choose your slicer");
+            let dialog = if cfg!(target_os = "windows") { dialog.add_filter("Programs", &["exe"]) } else { dialog };
+            let future = dialog.pick_file();
+            Box::pin(async move { future.await.map(|p| p.path().to_string_lossy().into_owned()) })
+        })),
         pick_save: Some(Box::new(move |name: &str, exts: &[&str]| {
             let dialog = parent.clone().set_file_name(name);
             let dialog = if exts == [io::DESIGN_EXT, "json"] {