fix: redirect players if host leaves

[anjaped]

Problem: if host leaves, other players are left hanging in session

• Lobby: session was deleted, but joiners ignored it
• In-game: session wasn’t deleted, so joiners kept waiting forever

Fix: added onSessionDeleted()

• Detect deleted session: added onSessionDeleted(), triggered it when session disappears, redirect players to start screen
• Host cleans up properly: host now calls deleteSession() when leaving mid-game