Outdoor Exercise Facility–Based Integrative Mobile Health Intervention to Support Physical Activity, Mental Well-Being, and Exercise Self-Efficacy Among Older Adults With Prefrailty and Frailty in Hong Kong: Pilot Feasibility Randomized Controlled Trial Study - Janet Lok Chun Lee; Arnold Y L Wong; Peter H F Ng; S N Fu; Kenneth N K Fong; Andy S K Cheng; Karen Nga Kwan Lee; Rui Sun; Hao Yi Zhang; Rong Xiao (JMIR mHealth and uHealth)