fix(ribbon): highlight View Cube button by toggle state

NAVVCUBE is a command toggle, so it never set active_tool and is_active_tool never matched it. Add a show_viewcube flag to the ribbon (mirroring wireframe/ortho_mode) and special-case "NAVVCUBE" in is_active_tool to return the live show_viewcube state; ToggleViewCube now calls ribbon.set_viewcube.

src/app/update/mod.rs

@@ -1575,6 +1575,7 @@ impl OpenCADStudio {
             }
             Message::ToggleViewCube => {
                 self.show_viewcube ^= true;
+                self.ribbon.set_viewcube(self.show_viewcube);
                 Task::none()
             }
             Message::ToggleProperties => {