Part of Steam P2P #4

Lobby lifecycle

•  Implement create_lobby()
•  Implement destroy_lobby() and cleanup logic

Invite system

•  Implement open_invite_menu() (Steam overlay trigger)
•  Connect lobby_chat_update signal
•  Implement Steam.join_requested signal

Network / P2P

•  Implement kick_friend()
•  Implement P2P Handshake (after friend joins lobby, initiate the actual network packet exchange to start the game session)
•  Polishing: Handle "Soft Kick" on client side