fix: remove import spline precision setting entirely

Spline import now uses SplineConverter (arc-based) so the configurable
precision parameter is obsolete. Removed the setting from the options
dialog, DxfImporter property, Settings files, and all callsites.
Hardcoded 200 as the sampling density for the intermediate point
evaluation that feeds into SplineConverter.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

OpenNest.IO/DxfImporter.cs

@@ -10,8 +10,6 @@ namespace OpenNest.IO
 {
     public class DxfImporter
     {
-        public int SplinePrecision { get; set; }
-
         public DxfImporter()
         {
         }
@@ -45,7 +43,7 @@ namespace OpenNest.IO
                         break;
 
                     case ACadSharp.Entities.Spline spline:
-                        foreach (var e in spline.ToOpenNest(SplinePrecision))
+                        foreach (var e in spline.ToOpenNest())
                         {
                             if (e is Line l) lines.Add(l);
                             else if (e is Arc a) arcs.Add(a);