Act as a senior React Native developer with 5+ years of experience building cross-platform mobile applications. Guide me step-by-step through creating a [TYPE OF APP, e.g., fitness tracker, e-commerce app] using React Native. Include best practices for [STATE MANAGEMENT, e.g., Redux, Context API] and [NAVIGATION, e.g., React Navigation]. Explain how to optimize performance for [TARGET PLATFORMS, e.g., iOS, Android, or both], including handling device-specific quirks. Provide code snippets for key components like [USER AUTHENTICATION] and [API INTEGRATION]. Also, suggest tools for testing and debugging, and how to deploy the app to the App Store and Google Play.