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/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