Add form validation to KontoTable and KreditorTable components. Disable save button when form is invalid. Clean up console logs in TextHeaderWithFilter and dataUtils.

client/src/components/admin/KreditorTable.js

@@ -102,6 +102,13 @@ class KreditorTable extends Component {
     });
   };
 
+  isFormValid = () => {
+    const { formData } = this.state;
+    return formData.iban.trim() !== '' && 
+           formData.name.trim() !== '' && 
+           formData.kreditorId.trim() !== '';
+  };
+
   handleSave = async () => {
     const { editingKreditor, formData } = this.state;
     
@@ -270,7 +277,11 @@ class KreditorTable extends Component {
           </DialogContent>
           <DialogActions>
             <Button onClick={this.handleCloseDialog}>Abbrechen</Button>
-            <Button onClick={this.handleSave} variant="contained">
+            <Button 
+              onClick={this.handleSave} 
+              variant="contained"
+              disabled={!this.isFormValid()}
+            >
               Speichern
             </Button>
           </DialogActions>