ui: mesh in the background from the mesher forms

OK on a mesher's form ran the command on the window thread: octree hex
on the bolted assembly froze the window for over a minute, with no
progress and no way to stop it. The tet and hex meshers' forms now run
their command as a background job on a copy of the session, as opening
a file and solving do: the window keeps drawing with the job's name and
Cancel, the form closes when the mesh is in and shows the error when it
is not. ui.dialog {run} stays synchronous for programmatic callers.

The new test walks a first-time user's path: Open picks a SolidCraft
design, Tet Mesh opens its form, OK with the defaults meshes the part,
and a mapped hex mesh of a holed plate says why it does not fit.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NmAttHhUZTokgPeEJia8sm

crates/ui-egui/src/context.rs

@@ -118,7 +118,7 @@ fn form(app: &mut ModelApp, ui: &mut egui::Ui, t: &Tokens) {
     if cancel {
         app.dialog = None;
     } else if run {
-        let _ = app.run_dialog();
+        app.submit_dialog();
     } else if use_selection
         && let Err(e) = app.use_selection_in_dialog()
         && let Some(d) = app.dialog.as_mut()