Add Avatar component to registry

[theoribbi]

Avatar Component (NativeUI)

What’s changed from the original idea:

• Split into three components (Avatar, AvatarImage, AvatarFallback) instead of a single monolith
• Switched to React Context for load/error state, removing manual flags and prop-drilling
• Added a configurable fallback delay (delayMs) to avoid flash-of-fallback
• Dropped explicit size and shape props in favor of utility classes via className (NativeWind)
• Mapped alt → accessibilityLabel for RN accessibility
• Ensured full TypeScript typings and forwardRef with displayName on each part