feat(ucs): dynamic-input deltas and angle in the active UCS

The on-canvas dynamic-input boxes measured relative X/Y deltas and the
polar angle in world axes. They now read in the active UCS plane (via
UcsXform::vec_to_ucs on the offset-invariant delta), matching the UCS
ortho/polar constraints and coordinate readout. Identity UCS reproduces
the world-frame values; absolute first-point fields are unchanged.

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

src/app/helpers.rs

@@ -110,7 +110,6 @@ impl UcsXform {
     }
 
     /// WCS direction → UCS (rotation only, no origin shift).
-    #[allow(dead_code)]
     pub(super) fn vec_to_ucs(&self, v: glam::Vec3) -> glam::Vec3 {
         glam::Vec3::new(v.dot(self.x), v.dot(self.y), v.dot(self.z))
     }