Add SenderAppType enum to represent sender application types

A new namespace `EnvelopeGenerator.Domain.Constants` was added,
containing the `SenderAppType` enumeration. This enum defines
two members: `LegacyFormApp` (0) and `ReceiverUIBlazorApp` (1).
The addition improves type safety and maintainability by
providing a structured way to differentiate between sender
application types.

EnvelopeGenerator.Domain/Constants/SenderAppType.cs

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