refactor: move material and thickness from Plate to Nest

Material and thickness are properties of the nest (all plates share the
same material/gauge), not individual plates. This moves them to the Nest
class, removes them from Plate and PlateSettings, and updates the UI so
EditNestInfoForm has a material field while EditPlateForm no longer shows
thickness. The nest file format gains top-level thickness/material fields
with backward-compatible reading from PlateDefaults for old files.

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

OpenNest/Controls/PlateView.cs

@@ -782,8 +782,6 @@ namespace OpenNest.Controls
             {
                 Quadrant = Plate.Quadrant,
                 PartSpacing = Plate.PartSpacing,
-                Thickness = Plate.Thickness,
-                Material = Plate.Material,
             };
             previewPlate.EdgeSpacing = Plate.EdgeSpacing;
             progressForm.PreviewPlate = previewPlate;