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/update/dialog.rs

@@ -597,7 +597,6 @@ pub(super) fn on_ribbon_tool_click(&mut self, tool_id: String, event: ModuleEven
                 match id {
                     PanelId::BlockPalette => {
                         self.show_block_palette = false;
-                        self.ribbon.set_block_palette(false);
                         self.block_palette.placing = None;
                     }
                     PanelId::Properties => {