refactor(ribbon): derive the Block Palette button highlight from app state

The ribbon kept its own \show_block_palette\ copy that the app had to
manually sync at every mutation point (the BLOCKPALETTE command and the
dock Close handler). Drop the copy and thread the app's authoritative
\show_block_palette\ into \Ribbon::view\ at render time, so the button
highlight can't drift from the panel's true visibility — those two
handlers are now mutate-points only and the ribbon view is the single
place that reads the state back.

src/app/view/mod.rs

@@ -1742,6 +1742,7 @@ impl OpenCADStudio {
                     self.tabs[self.active_tab].is_start,
                     self.tabs[self.active_tab].history.undo_stack.len(),
                     self.tabs[self.active_tab].history.redo_stack.len(),
+                    self.show_block_palette,
                 ));
             }
             if self.show_file_tabs {