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.Mcp/Tools/SetupTools.cs

@@ -31,8 +31,8 @@ namespace OpenNest.Mcp.Tools
             plate.Quadrant = quadrant;
             plate.Quantity = 1;
 
-            if (!string.IsNullOrEmpty(material))
-                plate.Material.Name = material;
+            if (!string.IsNullOrEmpty(material) && _session.Nest != null)
+                _session.Nest.Material = new Material(material);
 
             _session.Plates.Add(plate);