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/document.rs

@@ -186,6 +186,19 @@ impl DocumentTab {
         } else {
             Some(u)
         };
+        self.sync_ucs_to_scene();
+    }
+
+    /// Push the active UCS rotation into the scene so the ViewCube composes with
+    /// it. Call after any change to `active_ucs`.
+    pub(super) fn sync_ucs_to_scene(&mut self) {
+        let (_, x, y, z) = self.ucs_xform().axes();
+        self.scene.viewcube_ucs = glam::Mat4::from_cols(
+            x.extend(0.0),
+            y.extend(0.0),
+            z.extend(0.0),
+            glam::Vec4::W,
+        );
     }
 
     pub(super) fn new_drawing(n: usize) -> Self {