Add AccessCode feature to EnvelopeSenderPage UI

Introduced a new `AccessCode` property in `EnvelopeReceiverDto` to support displaying and copying access codes in the envelope sender page.

- Updated `EnvelopeReceiverDto` to include the nullable `AccessCode` property.
- Enhanced `EnvelopeSenderPage.razor` to display the `AccessCode` for each receiver, with a copy-to-clipboard button.
- Implemented `CopyAccessCodeAsync` for clipboard functionality using JavaScript interop.
- Styled the `AccessCode` section in `sender-page.css` for a responsive and user-friendly design.
- Improved error handling for clipboard operations with console warnings.

These changes improve the user experience by allowing senders to view and copy access codes for receivers.

EnvelopeGenerator.Application/Common/Dto/EnvelopeReceiver/EnvelopeReceiverDto.cs

@@ -60,6 +60,11 @@ public record EnvelopeReceiverDto
     /// </summary>
     public string? PrivateMessage { get; init; }
 
+    /// <summary>
+    /// 
+    /// </summary>
+    public string? AccessCode { get; init; }
+
     /// <summary>
     /// 
     /// </summary>
@@ -83,4 +88,4 @@ public record EnvelopeReceiverDto
         h.Receiver?.Id == ReceiverId && 
         h.Status == EnvelopeStatus.DocumentSigned
     ) ?? false;
-}
+}