User Experiences With Digital Future-Self Interventions in the Contexts of Smoking and Physical Inactivity: Mixed Methods Multistudy Exploration - Kristell M Penfornis; Nele Albers; Willem-Paul Brinkman; Mark A Neerincx; Andrea WM Evers; Winifred A Gebhardt; Eline Meijer (JMIR Formative Research)