Palaeoproteomics analysis of mollusc shells (Modiolus modiolus, Ostrea edulis, Unio pictorum, Margaritifera margaritifera, Unio crassus, Pseudunio auricularius) and shell ornaments (double-buttons) from Neolithic European sites (Havnø, Hornstaad-Hörnle IA, Peştera Ungurească).