refactor: rename Size.Height to Size.Length across codebase

"Length" is more natural than "height" for flat plate materials.
Renames the field on OpenNest.Geometry.Size, Box.Height property,
and all references across 38 files.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

OpenNest/Forms/EditNestInfoForm.cs

@@ -173,7 +173,7 @@ namespace OpenNest.Forms
                 return;
             }
 
-            if (TopSpacing + BottomSpacing >= size.Height)
+            if (TopSpacing + BottomSpacing >= size.Length)
             {
                 applyButton.Enabled = false;
                 return;