[Feature] Clean up session and players when leaving lobby

[odaakj]

Description

Currently, sessions and players are not properly removed from Firebase when users leave the lobby. This leads to:

• Sessions persisting even after the host exits the lobby
• Players remaining in a session after navigating away, allowing duplicate joins

This issue aims to ensure proper cleanup of sessions and players when leaving the lobby.

Requirements covered

• Ensure session lifecycle is correctly managed
• Ensure player list reflects only active participants

Acceptance criteria

Host leaves lobby

When the host exits the lobby:

• The entire session is deleted from Firestore
• All associated players are removed

Player leaves lobby

When a non-host player exits the lobby:

• The player is removed from Sessions/{sessionId}/Players
• The session remains active

Navigation handling

Leaving the lobby via:

• Back button
• Navigation to another screen
• Should trigger cleanup logic

Prevent duplicate players

A player should not be able to:

• Join the same session multiple times by navigating back and rejoining

Technical tasks

• Add removePlayerFromSession(...) to FirebaseGateway
• Add deleteSession(...) to FirebaseGateway
• Implement both in FirestoreSessionRepository
• Implement both in AndroidFirebaseGateway and NoOpFirebaseGateway
• Add leaveLobby(...) method in LobbyService
• Pass playerId and isHost to LobbyView / LobbyState
• Update LobbyController.onBackFromLobby(...) to trigger cleanup
• Ensure cleanup is only triggered once (avoid duplicate calls)

Related requirement

No response

Architecture layer

None