feat(ucs): interactive UCS icon with origin-snap grips, world-fixed compass

The UCS icon now anchors to the projected UCS origin when it is on-screen
(falling back to the bottom-left corner), and becomes a selectable object:
hover highlights it, a click shows draggable grips, and dragging the origin
grip relocates the UCS while the X/Y tip grips rotate it. Drags snap to
geometry/grid through the existing snapper, and the new UCS is persisted on
release. The icon stays selectable while parked in the corner.

Also make the ViewCube compass (N/E/S/W) world-fixed: the cube body still
reorients with the active UCS, but the cardinals project through a camera-only
rotation and snap in world frame, so the directions no longer spin with the UCS.

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

src/app/update/mod.rs

@@ -1079,6 +1079,7 @@ impl OpenCADStudio {
             }
 
             Message::ViewCubeSnap(region) => self.on_view_cube_snap(region),
+            Message::ViewCubeSnapWorld(region) => self.on_view_cube_snap_world(region),
 
             Message::ViewCubeHome => {
                 let i = self.active_tab;