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/frmEnvelopeMainData.vb

@@ -13,8 +13,8 @@ Public Class frmEnvelopeMainData
 
     Private Property EnvelopeTypeModel As EnvelopeTypeModel
 
-    <Bindable(True)>
-    Private Property EnvelopeType As New EnvelopeType
+    '<Bindable(True)>
+    'Private Property EnvelopeType As New EnvelopeType
 
     Public Property State As State
 
@@ -59,7 +59,7 @@ Public Class frmEnvelopeMainData
             cmbLanguage.Properties.Items.AddRange(New List(Of String) From {"de", "en"})
 
             groupAllOptions.Expanded = False
-            Dim EnvelopeType = oTypes.FirstOrDefault()
+            EnvelopeType = oTypes.FirstOrDefault()
             If NewEnvelopeMode = True Then
 
 
@@ -139,8 +139,8 @@ Public Class frmEnvelopeMainData
         Envelope.FinalEmailToCreator = cmbEmailToCreator.SelectedIndex
         Envelope.FinalEmailToReceivers = cmbEmailToReceivers.SelectedIndex
         Envelope.TFA_Enabled = chked_2Faktor.EditValue
-        ' ContractType kann zzt nicht über die Oberfläche gesetzt werden
-        Envelope.ContractType = ContractType.Contract
+        ' ContractType kann zzt nicht über die Oberfläche gesetzt werden - jetzt sollte es gehen.
+        Envelope.EnvelopeType = EnvelopeType
     End Sub
 
     Private Sub frmEnvelopeMainData_Shown(sender As Object, e As EventArgs) Handles Me.Shown