Refactor signature handling with SignatureCaptureDto

Introduced a new `SignatureCaptureDto` model to encapsulate
signature-related data and metadata, replacing the previous
`SignatureCapture` type in `EnvelopeViewer.razor`.

- Added `SignatureCaptureDto` in `SignatureCaptureDto.cs` with
  properties for signature image, signer name, position, and place.
- Updated `_capturedSignature` to use `SignatureCaptureDto` for
  consistency and maintainability.
- Refactored signature capture logic to initialize
  `SignatureCaptureDto` using object initializer syntax.
- Improved code clarity with detailed XML documentation for
  `SignatureCaptureDto`.

These changes enhance maintainability, readability, and ensure
a centralized model for signature-related data.

EnvelopeGenerator.ReceiverUI/Pages/EnvelopeViewer.razor

@@ -506,8 +506,7 @@ int _unsignedSignatures = 0;
 int _currentSignatureIndex = 0;  // Şu an hangi imzada (1-based)
 
 // Signature state
-record SignatureCapture(string DataUrl, string FullName, string Position, string Place);
-SignatureCapture? _capturedSignature;
+SignatureCaptureDto? _capturedSignature;
 bool _signaturePopupVisible = false;
 string? _popupValidationMessage;
 string _activeSignatureTab = SignatureTabDraw;
@@ -873,7 +872,13 @@ const int MaxThumbnailWidth = 400;
         }
 
         _popupValidationMessage = null;
-        _capturedSignature = new(signatureDataUrl, _signerFullName.Trim(), _signerPosition.Trim(), _signaturePlace.Trim());
+        _capturedSignature = new SignatureCaptureDto
+        {
+            DataUrl = signatureDataUrl,
+            FullName = _signerFullName.Trim(),
+            Position = _signerPosition.Trim(),
+            Place = _signaturePlace.Trim()
+        };
         _signaturePopupVisible = false;
         
         await InvokeAsync(StateHasChanged);