Refactor envelope type handling and validation logic

- Change ContractType property to use Constants.ContractType enum for improved type safety.
- Move EnvelopeType selection to a shared module-level variable for consistent access.
- Allow user selection of contract type when creating envelopes.
- Refine envelope validation to check for required elements per receiver, with improved error messages.

EnvelopeGenerator.Form/ModuleSettings.vb

@@ -27,4 +27,5 @@ Module ModuleSettings
     Public MyState As State
     Public CurrentEnvelopID As Integer = 0
     Public CurrentEnvelopetitle As String = ""
+    Public EnvelopeType As EnvelopeType
 End Module