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/DocumentDto.cs

@@ -31,5 +31,5 @@ public class DocumentDto
     /// <summary>
     /// Gets or sets the collection of elements associated with the document for receiver interactions, if any.
     /// </summary>
-    public IEnumerable<SignatureDto>? Elements { get; set; }
+    public IEnumerable<DocReceiverElementDto>? Elements { get; set; }
 }