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.IO/NestFormat.cs

@@ -24,6 +24,8 @@ namespace OpenNest.IO
             public string DateLastModified { get; init; } = "";
             public string Notes { get; init; } = "";
             public string AssistGas { get; init; } = "";
+            public double Thickness { get; init; }
+            public MaterialDto Material { get; init; } = new();
             public PlateDefaultsDto PlateDefaults { get; init; } = new();
             public List<DrawingDto> Drawings { get; init; } = new();
             public List<PlateDto> Plates { get; init; } = new();
@@ -57,11 +59,9 @@ namespace OpenNest.IO
         {
             public int Id { get; init; }
             public SizeDto Size { get; init; } = new();
-            public double Thickness { get; init; }
             public int Quadrant { get; init; } = 1;
             public int Quantity { get; init; } = 1;
             public double PartSpacing { get; init; }
-            public MaterialDto Material { get; init; } = new();
             public SpacingDto EdgeSpacing { get; init; } = new();
             public double GrainAngle { get; init; }
             public List<PartDto> Parts { get; init; } = new();

OpenNest.IO/NestReader.cs

@@ -180,13 +180,16 @@ namespace OpenNest.IO
             nest.Notes = dto.Notes;
             nest.AssistGas = dto.AssistGas ?? "";
 
-            // Plate defaults
+            // Nest-level material and thickness (fall back to PlateDefaults for old files)
             var pd = dto.PlateDefaults;
+            var matDto = dto.Material ?? pd.Material;
+            nest.Thickness = dto.Thickness > 0 ? dto.Thickness : pd.Thickness;
+            nest.Material = new Material(matDto.Name, matDto.Grade, matDto.Density);
+
+            // Plate defaults
             nest.PlateDefaults.Size = new OpenNest.Geometry.Size(pd.Size.Width, pd.Size.Length);
-            nest.PlateDefaults.Thickness = pd.Thickness;
             nest.PlateDefaults.Quadrant = pd.Quadrant;
             nest.PlateDefaults.PartSpacing = pd.PartSpacing;
-            nest.PlateDefaults.Material = new Material(pd.Material.Name, pd.Material.Grade, pd.Material.Density);
             nest.PlateDefaults.EdgeSpacing = new Spacing(pd.EdgeSpacing.Left, pd.EdgeSpacing.Bottom, pd.EdgeSpacing.Right, pd.EdgeSpacing.Top);
 
             // Drawings
@@ -198,11 +201,9 @@ namespace OpenNest.IO
             {
                 var plate = new Plate();
                 plate.Size = new OpenNest.Geometry.Size(p.Size.Width, p.Size.Length);
-                plate.Thickness = p.Thickness;
                 plate.Quadrant = p.Quadrant;
                 plate.Quantity = p.Quantity;
                 plate.PartSpacing = p.PartSpacing;
-                plate.Material = new Material(p.Material.Name, p.Material.Grade, p.Material.Density);
                 plate.EdgeSpacing = new Spacing(p.EdgeSpacing.Left, p.EdgeSpacing.Bottom, p.EdgeSpacing.Right, p.EdgeSpacing.Top);
                 plate.GrainAngle = p.GrainAngle;
 

OpenNest.IO/NestWriter.cs

@@ -79,6 +79,13 @@ namespace OpenNest.IO
                 DateLastModified = nest.DateLastModified.ToString("o"),
                 Notes = nest.Notes ?? "",
                 AssistGas = nest.AssistGas ?? "",
+                Thickness = nest.Thickness,
+                Material = new MaterialDto
+                {
+                    Name = nest.Material.Name ?? "",
+                    Grade = nest.Material.Grade ?? "",
+                    Density = nest.Material.Density
+                },
                 PlateDefaults = BuildPlateDefaultsDto(),
                 Drawings = BuildDrawingDtos(),
                 Plates = BuildPlateDtos()
@@ -91,14 +98,14 @@ namespace OpenNest.IO
             return new PlateDefaultsDto
             {
                 Size = new SizeDto { Width = pd.Size.Width, Length = pd.Size.Length },
-                Thickness = pd.Thickness,
+                Thickness = nest.Thickness,
                 Quadrant = pd.Quadrant,
                 PartSpacing = pd.PartSpacing,
                 Material = new MaterialDto
                 {
-                    Name = pd.Material.Name ?? "",
-                    Grade = pd.Material.Grade ?? "",
-                    Density = pd.Material.Density
+                    Name = nest.Material.Name ?? "",
+                    Grade = nest.Material.Grade ?? "",
+                    Density = nest.Material.Density
                 },
                 EdgeSpacing = new SpacingDto
                 {
@@ -196,16 +203,9 @@ namespace OpenNest.IO
                 {
                     Id = i + 1,
                     Size = new SizeDto { Width = plate.Size.Width, Length = plate.Size.Length },
-                    Thickness = plate.Thickness,
                     Quadrant = plate.Quadrant,
                     Quantity = plate.Quantity,
                     PartSpacing = plate.PartSpacing,
-                    Material = new MaterialDto
-                    {
-                        Name = plate.Material.Name ?? "",
-                        Grade = plate.Material.Grade ?? "",
-                        Density = plate.Material.Density
-                    },
                     EdgeSpacing = new SpacingDto
                     {
                         Left = plate.EdgeSpacing.Left,