feat(ui): add nested dimensions and area to progress window

Show width x length and total part area on the "Nested:" row
in the nesting progress dialog, using the existing GetBoundingBox
extension to compute the extents.

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

OpenNest.Engine/NestProgress.cs

@@ -37,6 +37,9 @@ namespace OpenNest
         public int PlateNumber { get; set; }
         public int BestPartCount { get; set; }
         public double BestDensity { get; set; }
+        public double NestedWidth { get; set; }
+        public double NestedLength { get; set; }
+        public double NestedArea { get; set; }
         public double UsableRemnantArea { get; set; }
         public List<Part> BestParts { get; set; }
         public string Description { get; set; }