feat(ribbon): contextual Block Editor tab while editing a block

Opening the block editor brings up a Block Editor ribbon tab with
Edit / Save / Discard, Attribute Definition, and the geometric,
dimensional and parameter tools that drive parametric blocks. The tab
is only listed while a block editor session is open, and leaving the
session returns to the tab that was active before.

Refs #509

Co-Authored-By: Claude Opus 5.5 <[email protected]>

src/app/update/mod.rs

@@ -339,6 +339,11 @@ impl OpenCADStudio {
             self.sync_underlay_tab();
         }
         self.sync_frame_dropdown();
+        let block_editing = self
+            .tabs
+            .get(self.active_tab)
+            .is_some_and(|tab| tab.active_block_edit.is_some());
+        self.ribbon.set_block_editing(block_editing);
         task
     }