refactor: Replace SupplierStock with StockItem/SupplierOffering model

- Remove SupplierStock entity
- Update Material navigation from SupplierStocks to StockItems
- Update Supplier navigation from Stocks to Offerings
- Update ApplicationDbContext with new DbSets and configurations
- Add unique constraints: StockItem(MaterialId, LengthInches) and
  SupplierOffering(SupplierId, StockItemId)

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

CutList.Web/Data/Entities/Supplier.cs

@@ -9,5 +9,5 @@ public class Supplier
     public bool IsActive { get; set; } = true;
     public DateTime CreatedAt { get; set; } = DateTime.UtcNow;
 
-    public ICollection<SupplierStock> Stocks { get; set; } = new List<SupplierStock>();
+    public ICollection<SupplierOffering> Offerings { get; set; } = new List<SupplierOffering>();
 }