feat(bedit): add Edit Block Definition dialog and new block authoring

Open the Edit Block Definition dialog when BEDIT is invoked without an active selection, listing candidate block definitions alphabetically while filtering layout records, anonymous blocks, and external references. Allow typing a new valid block name to start an empty block editor session that only commits to the drawing block table upon explicit save.

src/ui/window/mod.rs

@@ -5,6 +5,7 @@ pub mod attdef_dialog;
 pub mod attribute_editor;
 pub mod auto_constrain_settings;
 pub mod block_definition;
+pub mod block_edit_picker;
 pub mod block_save_as;
 pub mod block_palette;
 pub mod browser;