Fix date field display with readonly TextInput widget

- Changed from disabled DateInput to readonly TextInput for auto-generated payments
- Uses German date format (dd.mm.yyyy) for better user experience
- Added visual styling to indicate readonly state
- Preserves original date value through clean() method

app/stiftung/forms.py

@@ -998,18 +998,27 @@ class DestinataerUnterstuetzungForm(forms.ModelForm):
     def __init__(self, *args, **kwargs):
         super().__init__(*args, **kwargs)
         
-        # Make faellig_am disabled for automatically generated quarterly payments
+        # Make faellig_am read-only 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.is_auto_generated = True
-                self.fields['faellig_am'].disabled = True
+                
+                # Use a TextInput widget with readonly attribute to display the date
+                from django import forms
+                current_date = self.instance.faellig_am
+                if current_date:
+                    self.fields['faellig_am'].widget = forms.TextInput(
+                        attrs={
+                            "class": "form-control", 
+                            "readonly": True,
+                            "value": current_date.strftime('%d.%m.%Y'),  # German date format
+                            "style": "background-color: #f8f9fa; cursor: not-allowed;"
+                        }
+                    )
+                    self.fields['faellig_am'].initial = current_date
+                    
                 self.fields['faellig_am'].help_text = "Fälligkeitsdatum wird automatisch basierend auf Quartal berechnet"
-                # Ensure the initial value is set for disabled field and widget shows the value
-                self.fields['faellig_am'].initial = self.instance.faellig_am
-                self.fields['faellig_am'].widget.attrs.update({
-                    'value': self.instance.faellig_am.strftime('%Y-%m-%d') if self.instance.faellig_am else ''
-                })
     
     def clean(self):
         cleaned_data = super().clean()