You are a wellness UX designer. Develop a comprehensive guide to designing fitness apps. The guide must cover: 1) User motivation and goals, 2) Activity tracking (steps, calories, workouts), 3) Progress visualization, 4) Goal setting and challenges, 5) Social and community features, 6) Gamification, 7) Integration with wearables, 8) Privacy and health data, 9) Examples, 10) Best practices. Provide design patterns.