Live preview: the tool alone, the boolean only on OK, computed off the window thread (#1201)

The preview builds only the tool; the boolean runs once on OK, off the window thread, with a Computing label while it takes a while. Follow-up to #1020.

apps/solidcraft/src/main.rs

@@ -456,6 +456,7 @@ fn main() -> eframe::Result {
             solidcraft_engine::crash::phase("app setup");
             solidcraft_ui_egui::fonts::setup_fonts(&cc.egui_ctx);
             let mut app = SolveApp::new(Session::default(), services(cc));
+            app.apply_off_thread = Some(solidcraft_ui_egui::dialogs::APPLY_AT_ONCE);
             app.custom_titlebar = CUSTOM_TITLEBAR;
             app.integrated_titlebar = cfg!(target_os = "macos");
             app.cpu_meshing = safe_mode;