refactor(query): restructure ReceiverAlreadySignedQuery for clarity

- Replaced internal string properties with EnvelopeQuery and ReceiverQuery records
- Updated Key property to encode/decode using the new structured types
- Added overloaded IMediator extension methods IsSignedAsync for better usability
- Simplified ReceiverAlreadySignedQueryHandler to work with the new structure

EnvelopeGenerator.Web/Controllers/HomeController.cs

@@ -381,7 +381,7 @@ public class HomeController : ViewControllerBase
                     if(!isExisting)
                         return this.ViewEnvelopeNotFound();
 
-                    var signed = await _mediator.ReceiverAlreadySigned(envelopeReceiverId, cancel);
+                    var signed = await _mediator.IsSignedAsync(envelopeReceiverId, cancel);
                     if (signed)
                         return base.Redirect($"/EnvelopeKey/{envelopeReceiverId}/Locked");