feat(block-editor): add Test Block preview

Preview the current unsaved block in a disposable drawing with independent history. Keep normal Draw tools available, append Close Test Block, and return to the original editor without saving. Guard persistence paths and handle refresh and parent-close lifecycle by stable document identity.

src/app/update/dialog.rs

@@ -22,6 +22,9 @@ use iced::{mouse, Point, Task};
 
 impl OpenCADStudio {
     pub(in crate::app) fn open_save_dialog_window(&mut self, tab_idx: usize) -> Task<Message> {
+        if self.refuse_block_test_save(tab_idx) {
+            return Task::none();
+        }
         // Every entry here is Save-As semantics (the SAVEAS command, the
         // recovery save-as, the native failure redirects): the user is
         // choosing a destination, so on the web the FS Access picker must