Align arc creation and editing behavior

src/modules/draw/modify/offset.rs

@@ -127,8 +127,8 @@ fn offset_circle(c: &CircleEnt, dist: f64, side_pt: Vec3) -> Option<EntityType>
 // ── Arc offset ─────────────────────────────────────────────────────────────
 
 fn offset_arc(a: &ArcEnt, dist: f64, side_pt: Vec3) -> Option<EntityType> {
-    let px = side_pt.x as f64;
-    let py = side_pt.y as f64;
+    let plane = crate::entities::curve::arc_curve(a).plane;
+    let [px, py] = plane.project(side_pt.as_dvec3().to_array())?;
     let dc = ((px - a.center.x).powi(2) + (py - a.center.y).powi(2)).sqrt();
 
     let new_r = if dc < a.radius {