feat: Support multi-material project parts

Each project part now references its own material, allowing a single
project to use multiple material types. Removes ProjectStockBin entity
since stock is now derived from material stock lengths.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

CutList.Web/Data/Entities/Material.cs

@@ -11,7 +11,8 @@ public class Material
     public DateTime? UpdatedAt { get; set; }
 
     public ICollection<SupplierStock> SupplierStocks { get; set; } = new List<SupplierStock>();
-    public ICollection<Project> Projects { get; set; } = new List<Project>();
+    public ICollection<MaterialStockLength> StockLengths { get; set; } = new List<MaterialStockLength>();
+    public ICollection<ProjectPart> ProjectParts { get; set; } = new List<ProjectPart>();
 
     public string DisplayName => $"{Shape} - {Size}";
 }