feat: add warranty tracking for items and GRN lines

- Enhanced ItemService to include warranty and warranty period months in item DTOs.
- Implemented validation for warranty periods in ItemService.
- Updated frontend item detail and new item pages to handle warranty selection and input.
- Added warranty number handling in GRN creation and detail pages.
- Introduced new GrnLineWarrantyNumber entity to capture warranty numbers for received items.
- Created Warranty enum and associated allowed months for warranty coverage.
- Updated validation logic for GRN lines to ensure warranty numbers are captured correctly.

Backend/ERPCore/Domain/Entities/GrnLine.cs

@@ -58,4 +58,11 @@ public class GrnLine
     public decimal LineTotal { get; set; }
 
     public HoldStatus HoldStatus { get; set; } = HoldStatus.Available;
+
+    /// <summary>
+    /// One row per received unit when <see cref="Item.Warranty"/> is
+    /// <see cref="Enums.Warranty.Warranty"/> — count must equal <see cref="Qty"/>.
+    /// Empty for a non-warranty item.
+    /// </summary>
+    public ICollection<GrnLineWarrantyNumber> WarrantyNumbers { get; set; } = new List<GrnLineWarrantyNumber>();
 }

Backend/ERPCore/Domain/Entities/GrnLineWarrantyNumber.cs

@@ -0,0 +1,20 @@
+namespace ERPCore.Domain.Entities;
+
+/// <summary>
+/// One warranty number captured against a single received unit of a warranty-tracked
+/// item (<see cref="Item.Warranty"/> = <see cref="Enums.Warranty.Warranty"/>). A GRN line
+/// for such an item must carry exactly <see cref="GrnLine.Qty"/> of these — one per unit —
+/// mirroring how a Serial-tracked item requires one serial per unit (docs/10 Part C.3).
+/// </summary>
+public class GrnLineWarrantyNumber
+{
+    public int GrnLineWarrantyNumberId { get; set; }
+
+    public int GrnLineId { get; set; }
+    public GrnLine? GrnLine { get; set; }
+
+    public string WarrantyNo { get; set; } = string.Empty;
+
+    /// <summary>Warranty coverage length in months, selected at receipt (e.g. 3/6/12/18).</summary>
+    public int WarrantyPeriodMonths { get; set; }
+}

Backend/ERPCore/Domain/Entities/Item.cs

@@ -38,6 +38,9 @@ public class Item
 
     public StockNature StockNature { get; set; }
     public TrackingMode TrackingMode { get; set; }
+    public Warranty Warranty { get; set; } = Warranty.NonWarranty;
+    /// <summary>Coverage length in months (see <see cref="Enums.WarrantyPeriods.AllowedMonths"/>) when <see cref="Warranty"/> is Warranty; null otherwise.</summary>
+    public int? WarrantyPeriodMonths { get; set; }
     public string? TaxClass { get; set; }
 
     /// <summary>

Backend/ERPCore/Domain/Enums/Warranty.cs

@@ -0,0 +1,17 @@
+namespace ERPCore.Domain.Enums;
+
+/// <summary>
+/// Whether an item is sold under warranty (FR-MD-01). Stored as a string, same
+/// convention as <see cref="StockNature"/> and <see cref="TrackingMode"/>.
+/// </summary>
+public enum Warranty
+{
+    NonWarranty,
+    Warranty
+}
+
+/// <summary>Allowed warranty coverage lengths, in months — set once on the item (FR-MD-01).</summary>
+public static class WarrantyPeriods
+{
+    public static readonly int[] AllowedMonths = { 3, 6, 12, 18 };
+}