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/commands/blocks.rs

@@ -486,7 +486,6 @@ impl OpenCADStudio {
             // BLOCKPALETTE / BLOCKSPALETTE — toggle the docked Insert Block panel.
             "BLOCKPALETTE" | "BLOCKSPALETTE" => {
                 self.show_block_palette ^= true;
-                self.ribbon.set_block_palette(self.show_block_palette);
                 if self.show_block_palette {
                     // Always open expanded so the panel is immediately usable;
                     // the user can still collapse it via the pin (Auto) button.