Refactor signature processing in EnvelopeReceiverPageDataService

Refactored the logic to filter and map `elements` to `signatures`
before converting them to `UnitOfLength.Point`. Removed the direct
return of `elements` and ensured that only the processed `signatures`
are converted and returned. Added a `ToList()` call to materialize
the `signatures` collection before conversion.

EnvelopeGenerator.Server/EnvelopeGenerator.Server/Services/EnvelopeReceiverPageDataService.cs

@@ -45,11 +45,12 @@ public class EnvelopeReceiverPageDataService(
         if (document.Elements is not IEnumerable<DocReceiverElementDto> elements)
             return [];
 
-        return elements
+        var signatures = elements
             .Where(element => element.ReceiverId == receiverId)
             .Select(MapSignature)
-            .Convert(UnitOfLength.Point)
             .ToList();
+
+        return signatures.Convert(UnitOfLength.Point);
     }
 
     /// <summary>