Client-side (multiplayer.js):
- Add socket listeners for rematch events
- Update startMultiplayerGame to show surrender button and track opponent ID
- Update handleGameEnded to show game-over modal with stats
- Add handleRematchRequest method
- Add global helper functions for rematch acceptance/decline
Server-side (gameManager.js):
- Add rematches Map to track rematch requests
- Add handleSurrender method
- Add sendRematch, acceptRematch, declineRematch methods
- Handle surrender stats updates and game cleanup
Server (server.js):
- Add socket listeners for: surrender, send_rematch, accept_rematch, decline_rematch
Features now fully functional:
- Players can surrender during active games
- Players can request rematches after games end
- Opponents receive rematch notifications
- Game-over modal shows stats and rematch option
