Fix rotate manipulator axis and gizmo pivot placement

Rotate manipulator — correct axis bug:
- Object-space delta axis was set to initRot.GetRow3(axis) (a parent-space
  vector) then applied in the prim's local frame via deltaM * initRot.
  These frames are mismatched: the axis must be the canonical e_a so that
  Rot(e_a) fixes e_a and the prim rotates purely about its own axis.
  Fixed to use the canonical axis for both spaces; only composition order
  differs (deltaM * initRot for object, initRot * deltaM for world).

Gizmo pivot placement:
- ComputeGizmoPivot() used the bounding-box centre (geometry centroid),
  placing the gizmo at the wrong location for any non-centred prim.
  Replaced with parentToWorld.Transform(translate + pivot) from
  XformCommonAPI::GetXformVectors — the authored pivot point in world space.
  Fallback for incompatible op stacks uses worldMatrix.ExtractTranslation().

Property panel:
- Show pivot (X/Y/Z) as read-only row below Scale, populated from
  XformCommonAPI::GetXformVectors each frame.
- Removed active-rotate-axis display (was added and then removed per request).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/ui/ViewportPanel.h

@@ -65,6 +65,7 @@ public:
     LayoutMode GetLayout()           const { return m_layout;           }
     int        GetFocusedTileIndex() const { return m_focusedTileIndex; }
 
+
 private:
     // ── Tile rect helper ─────────────────────────────────────────────────────
     struct TileRect { ImVec2 pos; ImVec2 size; };