feat(grn): add update functionality for draft GRNs and support document-level discounts

- Implemented Update method in GrnsController to allow editing of draft GRNs.
- Enhanced CreateGrnRequest to include an optional totalDiscount property.
- Updated GrnService to handle GRN updates, including validation and line item processing.
- Modified NewGrnPage to support editing existing GRNs and applying document-level discounts.
- Improved UI in NewItemPage and GrnDetailPage for better user experience.
- Added search functionality to Select component for improved item selection.

Backend/ERPCore/Dtos/Grn/GrnDtos.cs

@@ -69,6 +69,8 @@ public sealed class CreateGrnRequest
     public int? VendorId { get; set; }
     [Required] public int WarehouseId { get; set; }
     [Required, MinLength(1)] public List<CreateGrnLineInput> Lines { get; set; } = new();
+    /// <summary>Optional document-level discount percentage (0–100). When supplied, per-line discounts are ignored.</summary>
+    [Range(0, 100)] public decimal? TotalDiscountPct { get; set; }
 }
 
 public sealed class ReleaseLineRequest