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/Domain/Entities/Item.cs

@@ -33,6 +33,14 @@ public class Item
     public StockNature StockNature { get; set; }
     public TrackingMode TrackingMode { get; set; }
     public string? TaxClass { get; set; }
+
+    /// <summary>
+    /// Optional fixed selling price used by Sales only. <c>null</c> means "use stock value"
+    /// (the item is sold at its FIFO stock cost at sale time); a value is the fixed sale price.
+    /// Never enters costing/GRN/FIFO (docs/10 Part C.1, C.9).
+    /// </summary>
+    public decimal? SalePrice { get; set; }
+
     public EntityStatus Status { get; set; } = EntityStatus.Active;
 
     public DateTime CreatedAt { get; set; }

Backend/ERPCore/Dtos/Items/ItemDtos.cs

@@ -9,7 +9,7 @@ namespace ERPCore.Dtos.Items;
 public sealed record ItemListItemDto(
     int ItemId, string Sku, string Name, int CategoryId, int? SubCategoryId, int? BrandId,
     int BaseUomId, int? DefaultVendorId, StockNature StockNature, TrackingMode TrackingMode,
-    string? TaxClass, EntityStatus Status);
+    string? TaxClass, decimal? SalePrice, EntityStatus Status);
 
 /// <summary>A single per-warehouse reorder policy row.</summary>
 public sealed record ItemReorderDto(int WarehouseId, decimal ReorderPoint, decimal ReorderQty);
@@ -27,7 +27,7 @@ public sealed record ItemDetailDto(
     int ItemId, string Sku, string Name, string? Description, int CategoryId,
     int? SubCategoryId, int? BrandId, int BaseUomId, int? DefaultVendorId,
     StockNature StockNature, TrackingMode TrackingMode,
-    string? TaxClass, EntityStatus Status, IReadOnlyList<ItemReorderDto> Reorder,
+    string? TaxClass, decimal? SalePrice, EntityStatus Status, IReadOnlyList<ItemReorderDto> Reorder,
     IReadOnlyList<UomConversionDto> Conversions,
     DateTime CreatedAt, DateTime? UpdatedAt);
 
@@ -62,6 +62,8 @@ public sealed class CreateItemRequest
     [Required, EnumDataType(typeof(StockNature))] public StockNature StockNature { get; set; }
     [EnumDataType(typeof(TrackingMode))] public TrackingMode TrackingMode { get; set; } = TrackingMode.None;
     [StringLength(20)] public string? TaxClass { get; set; }
+    /// <summary>Optional fixed sale price (Sales only). Null ⇒ sell at stock/FIFO value.</summary>
+    [Range(0, double.MaxValue)] public decimal? SalePrice { get; set; }
 }
 
 public sealed class UpdateItemRequest
@@ -79,6 +81,8 @@ public sealed class UpdateItemRequest
     [Required, EnumDataType(typeof(StockNature))] public StockNature StockNature { get; set; }
     [EnumDataType(typeof(TrackingMode))] public TrackingMode TrackingMode { get; set; } = TrackingMode.None;
     [StringLength(20)] public string? TaxClass { get; set; }
+    /// <summary>Optional fixed sale price (Sales only). Null ⇒ sell at stock/FIFO value.</summary>
+    [Range(0, double.MaxValue)] public decimal? SalePrice { get; set; }
 }
 
 public sealed class UpdateItemStatusRequest

Backend/ERPCore/Infra/Persistence/Configurations/ItemConfiguration.cs

@@ -19,6 +19,9 @@ public sealed class ItemConfiguration : IEntityTypeConfiguration<Item>
         builder.Property(i => i.Description).HasMaxLength(1000);
         builder.Property(i => i.TaxClass).HasMaxLength(20);
 
+        // Sales-only fixed selling price; nullable (null ⇒ sell at stock/FIFO value).
+        builder.Property(i => i.SalePrice).HasPrecision(18, 4);
+
         builder.Property(i => i.StockNature)
             .HasConversion<string>().HasMaxLength(20).IsRequired();
         builder.Property(i => i.TrackingMode)

Backend/ERPCore/Infra/Persistence/Migrations/ErpDbContextModelSnapshot.cs

@@ -385,6 +385,10 @@ namespace ERPCore.Infra.Persistence.Migrations
                         .HasColumnType("xid")
                         .HasColumnName("xmin");
 
+                    b.Property<decimal?>("SalePrice")
+                        .HasPrecision(18, 4)
+                        .HasColumnType("numeric(18,4)");
+
                     b.Property<string>("Sku")
                         .IsRequired()
                         .HasMaxLength(50)

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))