FRAILSURVEY—an mHealth App for Self-Assessment of Frailty Based on the Portuguese Version of the Groningen Frailty Indicator: Validation and Reliability Study - Luis Midao; Mafalda Duarte; Rute Sampaio; Marta Almada; Cláudia Camila Dias; Constança Paúl; Elísio Costa (JMIR Formative Research)