An app-based training for adolescents with problematic digital-media use and their parents (Res@t digital): protocol for a cluster-randomized clinical trial oleh Kerstin Paschke, Silke Diestelkamp, Antonia Zapf, Katharina Busch, Nicolas Arnaud, Alexander Prehn-Kristensen, Olaf Reis, Maria Stark, Jan-Ole Cloes, Anna-Lena Schulz, Hannah Brauer, Thomas Krömer, Rainer Thomasius, the Res@t Consortium