fix: reset editMode and form when navigating to vendor create page

frontend/src/pages/LieferantDetail.tsx

@@ -47,6 +47,14 @@ export default function LieferantDetail() {
   const [form, setForm] = useState<LieferantFormData>({ ...emptyForm });
   const [deleteDialogOpen, setDeleteDialogOpen] = useState(false);
 
+  // Sync edit mode when navigating between /neu and /:id within the same component instance
+  useEffect(() => {
+    if (isNew) {
+      setEditMode(true);
+      setForm({ ...emptyForm });
+    }
+  }, [isNew]);
+
   // ── Query ──
   const { data: vendor, isPending, isLoading, isError } = useQuery({
     queryKey: ['lieferant', vendorId],