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/Dtos/Sales/SalesReturnDtos.cs

@@ -0,0 +1,43 @@
+using System.ComponentModel.DataAnnotations;
+using ERPCore.Domain.Enums;
+
+namespace ERPCore.Dtos.Sales;
+
+// Responses -----------------------------------------------------------------
+
+public sealed record SalesReturnLineDto(int ReturnLineId, int? SalesInvoiceLineId, int ItemId, decimal Qty);
+
+public sealed record SalesReturnDto(
+    int ReturnId, string DocNo, int CustomerId, int WarehouseId, int ReasonCodeId, ReturnStatus Status,
+    int CreatedBy, DateTime CreatedAt, IReadOnlyList<SalesReturnLineDto> Lines, IReadOnlyList<int> LedgerRefs);
+
+/// <summary>Row shape for <c>GET /sales-returns</c> — no lines/ledgerRefs (those need a per-row query).</summary>
+public sealed record SalesReturnSummaryDto(
+    int ReturnId, string DocNo, int CustomerId, int WarehouseId, int ReasonCodeId, ReturnStatus Status,
+    int CreatedBy, DateTime CreatedAt, int LineCount, decimal TotalQty);
+
+/// <summary>
+/// Remaining returnable qty for one sales invoice line — the invoiced qty minus
+/// whatever has already been returned against it. The invoice line's own <c>Qty</c>
+/// is never mutated by a return, so this is computed on read from return history.
+/// </summary>
+public sealed record SalesInvoiceLineRemainingDto(int SalesInvoiceLineId, decimal RemainingQty);
+
+// Requests --------------------------------------------------------------------
+
+public sealed class CreateSalesReturnLineInput
+{
+    /// <summary>Original sales invoice line, for traceability against the sale.</summary>
+    public int? SalesInvoiceLineId { get; set; }
+    [Required] public int ItemId { get; set; }
+    [Range(0.0001, double.MaxValue)] public decimal Qty { get; set; }
+}
+
+public sealed class CreateSalesReturnRequest
+{
+    [Required] public int CustomerId { get; set; }
+    [Required] public int WarehouseId { get; set; }
+    /// <summary>Nullable so an omitted value is a distinct <c>REASON_CODE_REQUIRED</c> error.</summary>
+    public int? ReasonCodeId { get; set; }
+    [Required, MinLength(1)] public List<CreateSalesReturnLineInput> Lines { get; set; } = new();
+}