Block Definition dialog: preview, empty-block band, open in editor

The name card draws the selected objects, or the existing definition the
name picks. OK with no objects shows an in-window band offering to select
objects or to define an empty block. A switch opens the new block in the
block editor once it is defined; it is remembered with the other dialog
choices. Automation gains block_def_dialog.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/app/view/modal.rs

@@ -505,7 +505,7 @@ impl OpenCADStudio {
             }
             super::super::ModalKind::BlockDefinition => {
                 let state = self.block_definition.as_ref()?;
-                sized_flow(ex, 580, 390, |flow| {
+                sized_flow(ex, 600, 560, |flow| {
                     crate::ui::window::block_definition::view_window(state, flow)
                 })
             }