Add SenderAppType enum and integrate into SignatureDto

A new `SenderAppType` enum was introduced in the
`EnvelopeGenerator.ReceiverUI.Models.Constants` namespace, with
values `LegacyFormApp` and `ReceiverUIBlazorApp`.

The `SignatureDto` class was updated to include a new property of
type `SenderAppType`, enabling the specification of the sender
application type for a signature.

Additionally, a `using` directive for the
`EnvelopeGenerator.ReceiverUI.Models.Constants` namespace was added
to `SignatureDto.cs` to support the use of the new enum.

EnvelopeGenerator.ReceiverUI/Models/Constants/SenderAppType.cs

@@ -0,0 +1,8 @@
+namespace EnvelopeGenerator.ReceiverUI.Models.Constants
+{
+    public enum SenderAppType
+    {
+        LegacyFormApp = 0,
+        ReceiverUIBlazorApp = 1
+    }
+}

EnvelopeGenerator.ReceiverUI/Models/SignatureDto.cs

@@ -1,4 +1,6 @@
-namespace EnvelopeGenerator.ReceiverUI.Models;
+using EnvelopeGenerator.ReceiverUI.Models.Constants;
+
+namespace EnvelopeGenerator.ReceiverUI.Models;
 
 /// <summary>
 /// Represents a signature position on a PDF page.
@@ -19,4 +21,5 @@ public class SignatureDto
     /// <summary>1-based page number.</summary>
     public int Page { get; set; }
 
+    public SenderAppType SenderAppType { get; set; }
 }