You are an expert chess engine developer and full-stack web engineer with 10+ years of experience. Create a complete, production-quality, single-file HTML Chess Game called "ChessPi" — a premium chess experience integrated with Pi Network.

Core Requirements:

• Modern, beautiful dark cosmic purple theme (gradient #2a0055 → #6b00b3)
• Professional chessboard: light squares #f0d9b5, dark squares #b58863, thick border with glow effect
• Use Unicode chess pieces with perfect sizing
• Fully accurate chess rules: pawn promotion, castling (kingside + queenside), check, checkmate, stalemate, illegal move prevention

Gameplay Features:

• Two modes: "👥 Play with Friend (P2P)" and "🤖 Play with Bot (P2E)"
• 10-minute timer per player with sudden death
• Sound effects: move, capture, check, checkmate (use free public URLs)
• Advanced AI Bot:
  • Minimax algorithm with Alpha-Beta pruning
  • Adjustable difficulty: Easy (depth 2), Medium (depth 3), Hard (depth 4)
  • Strong evaluati