Fix form validation for disabled due date field

- Changed readonly to disabled for faellig_am field in auto-generated payments
- Added clean method to preserve original due date for auto-generated payments
- This prevents validation errors when editing auto-generated quarterly payments

app/stiftung/forms.py

@@ -998,11 +998,22 @@ class DestinataerUnterstuetzungForm(forms.ModelForm):
     def __init__(self, *args, **kwargs):
         super().__init__(*args, **kwargs)
         
-        # Make faellig_am read-only for automatically generated quarterly payments
+        # Make faellig_am disabled for automatically generated quarterly payments
+        self.is_auto_generated = False
         if self.instance and self.instance.pk and self.instance.beschreibung:
             if "Vierteljährliche Unterstützung" in self.instance.beschreibung and "(automatisch erstellt)" in self.instance.beschreibung:
-                self.fields['faellig_am'].widget.attrs['readonly'] = True
+                self.is_auto_generated = True
+                self.fields['faellig_am'].disabled = True
                 self.fields['faellig_am'].help_text = "Fälligkeitsdatum wird automatisch basierend auf Quartal berechnet"
+    
+    def clean(self):
+        cleaned_data = super().clean()
+        
+        # For auto-generated payments, preserve the original due date
+        if self.is_auto_generated and self.instance and self.instance.pk:
+            cleaned_data['faellig_am'] = self.instance.faellig_am
+            
+        return cleaned_data
 
 
 class DestinataerNotizForm(forms.ModelForm):