VSN-2095/ feat: dark mode persistence, dynamic greeting, and profile screen animation fix

Changes :

• Implemented persistent dark mode using AsyncStorage.
• Add home screen greeting with a time-based salutation followed by the child's first name. The profile is now fetched on home mount, so the name appears immediately.
• The fade-in animation was starting while the loading spinner was visible, causing the Animated.View to be unmounted during the animation. On remount, the view stayed stuck at opacity 0. Fixed by decoupling the animation start from fetchProfile() — the animation now only runs once isLoading is false and profile is available.