Refactor: Rename SignatureDto to DocReceiverElementDto

Renamed `SignatureDto` to `DocReceiverElementDto` across the codebase to better reflect its purpose as a DTO for document receiver elements.

Updated all references, including:
- `SignatureController.cs`: Changed `doc.Elements` type to `IEnumerable<DocReceiverElementDto>`.
- `DocumentDto.cs`: Updated `Elements` property type.
- `MappingProfile.cs`: Adjusted mappings for the renamed DTO.
- `IDocumentReceiverElementService.cs` and `DocumentReceiverElementService.cs`: Updated interfaces and services to use the new DTO.
- `TestDocumentReceiverElementController.cs`: Updated generic type parameters.

These changes improve clarity, align naming with the domain model, and ensure consistency throughout the application.

EnvelopeGenerator.Application/Common/Dto/DocReceiverElementDto.cs

@@ -0,0 +1,128 @@
+using EnvelopeGenerator.Domain.Constants;
+using EnvelopeGenerator.Domain.Interfaces;
+using Microsoft.AspNetCore.Mvc;
+using System.ComponentModel.DataAnnotations.Schema;
+
+namespace EnvelopeGenerator.Application.Common.Dto;
+
+/// <summary>
+/// Data Transfer Object representing a positioned element assigned to a document receiver.
+/// </summary>
+[ApiExplorerSettings(IgnoreApi = true)]
+public class DocReceiverElementDto : IDocReceiverElement
+{
+    /// <summary>
+    /// Gets or sets the unique identifier of the element.
+    /// </summary>
+    public int Id { get; set; }
+
+    /// <summary>
+    /// Gets or sets the identifier of the associated document.
+    /// </summary>
+    public int DocumentId { get; set; }
+
+    /// <summary>
+    /// Gets or sets the identifier of the receiver.
+    /// </summary>
+    public int ReceiverId { get; set; }
+
+    /// <summary>
+    /// Gets or sets the type of the element.
+    /// </summary>
+    public int ElementType { get; set; }
+
+    /// <summary>
+    /// Gets or sets the X coordinate of the element.
+    /// </summary>
+    public double X { get; set; }
+
+    /// <summary>
+    /// Gets or sets the Y coordinate of the element.
+    /// </summary>
+    public double Y { get; set; }
+
+    /// <summary>
+    /// Gets or sets the width of the element.
+    /// </summary>
+    public double Width { get; set; }
+
+    /// <summary>
+    /// Gets or sets the height of the element.
+    /// </summary>
+    public double Height { get; set; }
+
+    /// <summary>
+    /// Gets or sets the page number where the element appears.
+    /// </summary>
+    public int Page { get; set; }
+
+    /// <summary>
+    /// Gets or sets a value indicating whether the element is required.
+    /// </summary>
+    public bool Required { get; set; }
+
+    /// <summary>
+    /// Gets or sets the tooltip text for the element.
+    /// </summary>
+    public string? Tooltip { get; set; }
+
+    /// <summary>
+    /// Gets or sets a value indicating whether the element is read-only.
+    /// </summary>
+    public bool ReadOnly { get; set; }
+
+    /// <summary>
+    /// Gets or sets the annotation index for ordering or reference.
+    /// </summary>
+    public int AnnotationIndex { get; set; }
+
+    /// <summary>
+    /// Gets or sets the timestamp when the element was added.
+    /// </summary>
+    public DateTime AddedWhen { get; set; }
+
+    /// <summary>
+    /// Gets or sets the timestamp when the element was last changed, if applicable.
+    /// </summary>
+    public DateTime? ChangedWhen { get; set; }
+
+    /// <summary>
+    /// Gets or sets the top position of the element (in layout terms).
+    /// </summary>
+    public double Top => Y;
+
+    /// <summary>
+    /// Gets or sets the left position of the element (in layout terms).
+    /// </summary>
+    public double Left => X;
+
+    /// <summary>
+    /// 
+    /// </summary>
+    public IEnumerable<AnnotationDto>? Annotations { get; set; }
+
+    /// <summary>
+    /// 
+    /// </summary>
+    public SenderAppType SenderAppType { get; set; } = SenderAppType.LegacyFormApp;
+
+    /// <summary>
+    /// 
+    /// </summary>
+    public string? FullName { get; set; }
+
+    /// <summary>
+    /// 
+    /// </summary>
+    public string? Position { get; set; }
+
+    /// <summary>
+    /// 
+    /// </summary>
+    public string? Place { get; set; }
+
+    /// <summary>
+    /// 
+    /// </summary>
+    public byte[]? Ink { get; set; }
+}