feat: Update service layer for new stock model

- Add StockItemService for CRUD operations on stock items
- Update SupplierService to manage SupplierOfferings instead of
  SupplierStock (GetOfferingsForSupplierAsync, AddOfferingAsync, etc.)
- Update CutListPackingService to use StockItems for available lengths
- Register StockItemService in Program.cs

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

CutList.Web/Program.cs

@@ -17,6 +17,7 @@ builder.Services.AddDbContext<ApplicationDbContext>(options =>
 // Add application services
 builder.Services.AddScoped<MaterialService>();
 builder.Services.AddScoped<SupplierService>();
+builder.Services.AddScoped<StockItemService>();
 builder.Services.AddScoped<ProjectService>();
 builder.Services.AddScoped<CutListPackingService>();
 builder.Services.AddScoped<ReportService>();