feat(procurement): enhance purchase order and GRN functionalities

- Updated purchase order descriptions for clarity on draft and submission processes.
- Implemented submit and delete functionalities for draft purchase orders, allowing users to manage their orders more effectively.
- Added discount and VAT fields to GRN lines, enabling better cost tracking and reporting.
- Enhanced validation for GRN lines to ensure discount and VAT percentages are within acceptable ranges.
- Updated API to support new functionalities, including submitting and deleting purchase orders.
- Improved UI components for better user experience in managing purchase orders and GRNs.
- Documented changes in security and backend phase documentation to reflect new processes and requirements.

Backend/ERPCore/Dtos/Grn/GrnDtos.cs

@@ -7,7 +7,10 @@ namespace ERPCore.Dtos.Grn;
 
 public sealed record GrnLineDto(
     int GrnLineId, int? PoLineId, int ItemId, int UomId, int? BinId,
-    decimal Qty, decimal UnitCost, decimal ReceivedValue, HoldStatus HoldStatus, int? BatchId);
+    decimal Qty, decimal UnitCost, decimal? PoUnitPrice,
+    decimal DiscountPct, decimal NetUnitCost, decimal VatPct, decimal VatAmount,
+    decimal ReceivedValue, decimal LineTotal, decimal PriceVariance,
+    HoldStatus HoldStatus, int? BatchId);
 
 public sealed record GrnDto(
     int GrnId, string DocNo, int? PoId, int VendorId, int WarehouseId, GrnStatus Status,
@@ -44,8 +47,16 @@ public sealed class CreateGrnLineInput
     [Required] public int UomId { get; set; }
     public int? BinId { get; set; }
     [Range(0.0001, double.MaxValue)] public decimal Qty { get; set; }
-    /// <summary>Used only for direct (no-PO) receipts; ignored when <see cref="PoLineId"/> is set.</summary>
+    /// <summary>
+    /// Gross unit cost. Required for direct (no-PO) receipts. For a PO line it is an optional
+    /// per-receipt price override — when 0/omitted the PO line price is used; when supplied it
+    /// wins and a variance is recorded against the PO snapshot (docs/02-SECURITY C.3, revised).
+    /// </summary>
     [Range(0, double.MaxValue)] public decimal UnitCost { get; set; }
+    /// <summary>Trade discount percentage (0–100). Reduces the inventory cost.</summary>
+    [Range(0, 100)] public decimal DiscountPct { get; set; }
+    /// <summary>VAT percentage (0–100). Recoverable — does not affect stock value.</summary>
+    [Range(0, 100)] public decimal VatPct { get; set; }
     [EnumDataType(typeof(HoldStatus))] public HoldStatus HoldStatus { get; set; } = HoldStatus.Available;
     public BatchInput? Batch { get; set; }
 }

Backend/ERPCore/Dtos/Procurement/PurchaseOrderDtos.cs

@@ -37,6 +37,13 @@ public sealed class CreatePurchaseOrderRequest
     [Required] public int VendorId { get; set; }
     public int? RequisitionId { get; set; }
     [Required, MinLength(1)] public List<CreatePoLineInput> Lines { get; set; } = new();
+
+    /// <summary>
+    /// When true the PO is created in <c>Draft</c> (editable/deletable, not yet issued).
+    /// When false (default) it auto-approves on creation, preserving the Requisition→PO
+    /// and RFQ→PO flows unchanged (docs/11 §3.3).
+    /// </summary>
+    public bool SaveAsDraft { get; set; }
 }
 
 public sealed class UpdatePurchaseOrderRequest