Refactor CreateEnvelopeCommand authorization method

Replaced the `Authorize` method with a new `WithAuth` method
in the `CreateEnvelopeCommand` class. The new method sets
the `UserId` property and returns the current instance,
enabling method chaining and improving usability.

EnvelopeGenerator.Application/Envelopes/Commands/CreateEnvelopeCommand.cs

@@ -40,10 +40,10 @@ public record CreateEnvelopeCommand : IRequest<EnvelopeDto?>
     /// </summary>
     /// <param name="userId"></param>
     /// <returns></returns>
-    public bool Authorize(int userId)
+    public CreateEnvelopeCommand WithAuth(int userId)
     {
         UserId = userId;
-        return true;
+        return this;
     }
 
     /// <summary>