feat: add sales return functionality

- Introduced Sales Return types and interfaces in the frontend for managing sales returns.
- Implemented SalesReturnsController in the backend to handle sales return endpoints.
- Created SalesReturn and SalesReturnLine entities to represent sales return data.
- Added DTOs for sales return responses and requests.
- Configured Entity Framework for SalesReturn and SalesReturnLine entities.
- Developed ISalesReturnService interface and its implementation for business logic.
- Added API methods for listing sales returns, creating new returns, and fetching remaining returnable quantities.
- Created frontend components for creating and listing sales returns, including validation logic.
- Implemented UI for selecting invoices, reason codes, and managing return lines.

Backend/ERPCore/Domain/DocumentTypes.cs

@@ -20,4 +20,5 @@ public static class DocumentTypes
     public const string SalesInvoice = "SI";
     public const string SalesSlip = "SSL";
     public const string BundleSale = "BND";
+    public const string SalesReturn = "SRET";
 }

Backend/ERPCore/Domain/Entities/SalesReturn.cs

@@ -0,0 +1,32 @@
+using ERPCore.Domain.Enums;
+
+namespace ERPCore.Domain.Entities;
+
+/// <summary>
+/// Sales return header — a customer returns previously sold goods, generating an
+/// inbound stock movement. Auto-posts with a mandatory reason code, mirroring
+/// <see cref="PurchaseReturn"/> with the direction reversed.
+/// </summary>
+public class SalesReturn
+{
+    public int ReturnId { get; set; }
+    public string DocNo { get; set; } = string.Empty;
+
+    public int CustomerId { get; set; }
+    public Customer? Customer { get; set; }
+
+    public int WarehouseId { get; set; }
+    public Warehouse? Warehouse { get; set; }
+
+    public int ReasonCodeId { get; set; }
+    public ReasonCode? ReasonCode { get; set; }
+
+    public ReturnStatus Status { get; set; } = ReturnStatus.Posted;
+
+    public int CreatedBy { get; set; }
+    public User? Creator { get; set; }
+
+    public DateTime CreatedAt { get; set; }
+
+    public ICollection<SalesReturnLine> Lines { get; set; } = new List<SalesReturnLine>();
+}

Backend/ERPCore/Domain/Entities/SalesReturnLine.cs

@@ -0,0 +1,21 @@
+namespace ERPCore.Domain.Entities;
+
+/// <summary>
+/// Sales-return line referencing the original sales invoice line for traceability.
+/// <see cref="Qty"/> is in base UOM.
+/// </summary>
+public class SalesReturnLine
+{
+    public int ReturnLineId { get; set; }
+
+    public int ReturnId { get; set; }
+    public SalesReturn? Return { get; set; }
+
+    public int? SalesInvoiceLineId { get; set; }
+    public SalesInvoiceLine? SalesInvoiceLine { get; set; }
+
+    public int ItemId { get; set; }
+    public Item? Item { get; set; }
+
+    public decimal Qty { get; set; }
+}