Add sales report functions

Backend/ERPCore/Program.cs

@@ -93,6 +93,7 @@ builder.Services.AddScoped<IGrnService, GrnService>();
 builder.Services.AddScoped<ISalesPricingService, SalesPricingService>();
 builder.Services.AddScoped<ISalesInvoiceService, SalesInvoiceService>();
 builder.Services.AddScoped<ISalesSlipService, SalesSlipService>();
+builder.Services.AddScoped<ISalesReportService, SalesReportService>();
 
 // Stock transactions + reference data (docs/11 §5–6)
 builder.Services.AddScoped<IStockMutator, StockMutator>();