fix(viewcube): hover/click hit-test must match the drawn cube under UCS

The hover hit-tests used the bare camera rotation (no UCS) and the main
camera even inside a floating viewport, so the highlighted face diverged
from the cursor whenever a UCS was active or in a paper viewport — while
the snap path was correct. Route hover, click-consume and snap through
active_view_rotation_mat(), and compose the ViewCube UCS in its viewport
branch so all three match ViewportData's drawn cam_rotation exactly.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

src/scene/mod.rs

@@ -3906,9 +3906,13 @@ impl Scene {
     /// paper-space camera's matrix when not in MSPACE.
     /// Used by ViewCube hit-testing so clicks map to the correct camera.
     pub fn active_view_rotation_mat(&self) -> glam::Mat4 {
+        // Must match exactly what the drawn cube uses (see ViewportData's
+        // `cam_rotation`): the active context's camera composed with the
+        // ViewCube UCS. Inside a floating viewport that's the viewport's own
+        // camera; the UCS factor applies in both cases.
         if let Some(h) = self.active_viewport {
             if let Some(cam) = self.camera_for_viewport(h) {
-                return cam.view_rotation_mat();
+                return cam.view_rotation_mat() * self.viewcube_ucs_mat();
             }
         }
         self.camera.borrow().view_rotation_mat() * self.viewcube_ucs_mat()