feat: implement React-based ChessBoard to replace legacy ChessClient in LessonOverlay

[F-Hejazi]

This work introduces a fully React-based ChessBoard component and integrates it into LessonOverlay, replacing the previous iframe-dependent ChessClient setup.

Key changes:

• Removed iframe and postMessage logic
• Added prop-based callbacks (onMove, onReset, onPromote) for interaction with LessonOverlay
• ChessBoard component handles move validation, undo, pawn promotion, and visual feedback using chess.js
• Integrated game status notifications (check, checkmate, draw)
• Exposed board orientation, reset, and lesson move tracking via useImperativeHandle

This implementation lays the foundation for future integrations, including live mentor-student gameplay and real-time lesson progress tracking.

[khushita28]

Nice work on this refactor. The ChessBoard integration feels smooth and way cleaner than the old iframe setup. The hooks make the logic easy to follow, and the state handling looks solid.