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.API/Controllers/SignatureController.cs

@@ -44,7 +44,7 @@ public class SignatureController : ControllerBase
 
         var doc = await _mediator.Send(new ReadDocumentQuery() { EnvelopeId = envelopeId }, cancel);
 
-        if (doc.Elements is not IEnumerable<SignatureDto> docSignatures)
+        if (doc.Elements is not IEnumerable<DocReceiverElementDto> docSignatures)
             return NotFound("Document is empty.");
 
         var rcvSignatures = docSignatures.Where(s => s.ReceiverId == receiverId).ToList();