feat(ucs): orient the ViewCube to the active UCS

The ViewCube showed world Front/Top/Right. It now composes the camera
rotation with the active UCS (scene.viewcube_ucs, synced from the tab's
active_ucs), so its faces follow the user's coordinate system in model
space. Render uniform, hover/click hit-test, and the click→snap direction
all go through scene.viewcube_ucs_mat() so they stay in lock-step;
identity outside model space leaves paper-viewport navigation unchanged.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/commands.rs

@@ -3822,6 +3822,8 @@ impl OpenCADStudio {
                         }
                     }
                 }
+                // Keep the scene's ViewCube UCS in lock-step with active_ucs.
+                self.tabs[i].sync_ucs_to_scene();
             }
 
             // ── Named Views (VIEW command) ────────────────────────────────