Refactor entities: remove preprocessor blocks, unify style

Refactored Envelope, Receiver, and Signature entity classes to eliminate preprocessor directives around class and property definitions. Applied attributes directly and reformatted properties for clarity. Constructors are now always present, with default initializations still conditional where needed. Removed obsolete directives from IHasAddedWho. These changes improve code readability, maintainability, and cross-platform compatibility.

EnvelopeGenerator.Domain/Interfaces/IHasAddedWho.cs

@@ -1,7 +1,3 @@
-#if NETFRAMEWORK
-using System;
-#endif
-
 namespace EnvelopeGenerator.Domain.Interfaces
 {
     public interface IHasAddedWho