feat: Implement fixed sale price functionality for items

- Added a toggle for fixed sale price vs stock value in the item creation form.
- Introduced validation to ensure all variants have a price greater than 0 when fixed price mode is selected.
- Updated the item model to include a nullable salePrice field, which is used for sales only and does not affect GRN/FIFO/ledger.
- Enhanced the GRN page to allow off-PO items and included a refresh button to update the item list without reloading the page.
- Updated documentation to reflect changes in item pricing and GRN handling.

Backend/ERPCore/Services/ItemService.cs

@@ -79,7 +79,7 @@ public sealed class ItemService : IItemService
             .Select(i => new ItemListItemDto(
                 i.ItemId, i.Sku, i.Name, i.CategoryId, i.SubCategoryId, i.BrandId,
                 i.BaseUomId, i.DefaultVendorId,
-                i.StockNature, i.TrackingMode, i.TaxClass, i.Status))
+                i.StockNature, i.TrackingMode, i.TaxClass, i.SalePrice, i.Status))
             .ToListAsync(ct);
 
         return PagedResponse<ItemListItemDto>.Create(rows, query.Page, query.PageSize, total);
@@ -117,6 +117,7 @@ public sealed class ItemService : IItemService
             StockNature = request.StockNature,
             TrackingMode = request.TrackingMode,
             TaxClass = request.TaxClass,
+            SalePrice = request.SalePrice,
             Status = EntityStatus.Active,
             CreatedAt = DateTime.UtcNow
         };
@@ -158,6 +159,7 @@ public sealed class ItemService : IItemService
         item.StockNature = request.StockNature;
         item.TrackingMode = request.TrackingMode;
         item.TaxClass = request.TaxClass;
+        item.SalePrice = request.SalePrice;
         item.UpdatedAt = DateTime.UtcNow;
 
         await SaveGuardingConcurrencyAsync(ct);
@@ -348,7 +350,7 @@ public sealed class ItemService : IItemService
     private static ItemDetailDto ToDetail(Item i) => new(
         i.ItemId, i.Sku, i.Name, i.Description, i.CategoryId, i.SubCategoryId, i.BrandId,
         i.BaseUomId, i.DefaultVendorId,
-        i.StockNature, i.TrackingMode, i.TaxClass, i.Status,
+        i.StockNature, i.TrackingMode, i.TaxClass, i.SalePrice, i.Status,
         i.ReorderSettings
             .OrderBy(r => r.WarehouseId)
             .Select(r => new ItemReorderDto(r.WarehouseId, r.ReorderPoint, r.ReorderQty))