Merge PR #353: fix View Cube toggle (highlight follows state, cube clears on hide)

External PR by Karim Jerbi (@KarimJerbi).
- NAVVCUBE is a command toggle so it never set active_tool; add a
  show_viewcube flag to the ribbon and special-case is_active_tool so the
  button highlights only while the cube is on.
- show_viewcube was absent from render_signature, so toggling a cached
  view produced an identical signature and left stale cube pixels; hash
  it into the signature so the toggle invalidates the cache and clears.

src/app/update/mod.rs

@@ -1562,6 +1562,7 @@ impl OpenCADStudio {
             }
             Message::ToggleViewCube => {
                 self.show_viewcube ^= true;
+                self.ribbon.set_viewcube(self.show_viewcube);
                 Task::none()
             }
             Message::ToggleProperties => {