fix(viewcube): snap a paper viewport square to the UCS, not WCS

Clicking the ViewCube inside a floating viewport set only view_direction,
so the rebuilt camera fell back to WCS-up and the view opened tilted
relative to the UCS-oriented cube the user clicked. Build the full
UCS-aligned orientation (snap_to_direction with the ViewCube UCS) and
decode it to the viewport's stored (view_direction, twist_angle).

twist can't come from -Camera::roll(): camera_from_view rebuilds rotation
with its own yaw convention (atan2(x,-y)) and roll = -twist, which differs
from the camera's internal sync_yaw_pitch. Instead reproduce the decoder's
zero-twist basis and store twist = -signed_angle(up0 -> desired_up).

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

src/app/update.rs

@@ -4433,7 +4433,7 @@ impl OpenCADStudio {
                 if self.tabs[i].scene.active_viewport.is_some() {
                     self.tabs[i]
                         .scene
-                        .snap_active_viewport_to_direction(eye_dir);
+                        .snap_active_viewport_to_direction(eye_dir, r_ucs);
                 } else {
                     let mut cam = self.tabs[i].scene.camera.borrow_mut();
                     cam.snap_to_direction(eye_dir, r_ucs);