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/ProjectPart.cs

@@ -4,10 +4,12 @@ public class ProjectPart
 {
     public int Id { get; set; }
     public int ProjectId { get; set; }
+    public int MaterialId { get; set; }
     public string Name { get; set; } = string.Empty;
     public decimal LengthInches { get; set; }
     public int Quantity { get; set; } = 1;
     public int SortOrder { get; set; }
 
     public Project Project { get; set; } = null!;
+    public Material Material { get; set; } = null!;
 }