Refactor to use SignatureDto and SignatureService

Replaced AnnotationDto and AnnotationService with SignatureDto
and SignatureService for handling signature data. Marked
AnnotationDto and AnnotationService as obsolete.

Added the SignatureDto class to represent signature data and
introduced the SignatureService class to fetch signature data
from the API. Updated EnvelopeViewer.razor to use
SignatureService, replacing AnnotationService, and added a
debug log for retrieved signatures.

Performed general refactoring to align with the new signature
data model and functionality.

EnvelopeGenerator.ReceiverUI/Models/AnnotationDto.cs

@@ -13,6 +13,7 @@ namespace EnvelopeGenerator.ReceiverUI.Models;
 /// <b>Difference from GDPicture:</b> GDPicture uses PDF points with <b>bottom-left</b> origin (PDF standard); Y is flipped.
 /// Convert: <c>yDX = (pageHeightPt - yGD - elemHeightPt) * (100.0 / 72.0)</c>
 /// </summary>
+[Obsolete("Use SignatureDto with SignatureService.")]
 public record AnnotationDto
 {
     /// <summary>Unique identifier of the annotation.</summary>