Light Mode Theme for Better UX

[devAmaresh]

Description:
SkinSenseAI currently uses a fixed theme. Adding a system-based dark mode/light mode toggle would improve accessibility and usability, especially for users who prefer different visual modes.

Proposed Solution:

Implement theme context in React Native frontend.

Provide a toggle option in app settings.

Ensure consistent color palette mapping across screens (using NativeWind and theme tokens).