54 clean up architecture and fix bugs

[odaakj]

What does this PR do?

Reorganizes the project architecture to better reflect the intended MVC + Client-Server + ECS separation, and fixes several bugs discovered during the cleanup.

Architecture changes:

• Splits model package into domain/, dto/, score/, and services/ subpackages
• Extracts QuestionBuilder, SessionSyncService, and RoundManager from LobbyController/RoundController to give each class a single clear responsibility

Bug fixes:

• Music no longer continues playing when the host leaves a session mid-round
• Music no longer continues playing when a regular player leaves mid-round
• Music now stops correctly for all players on transition from game round to leaderboard
• Fixed leaderboard showing the previous round's score delta (+X) for players who answered wrong — they now correctly show +0
• Fixed leaderboard not showing the correct round score for the local player when answering correctly
• Firebase listeners are now stopped in BeatBattle.dispose(), preventing a listener leak when the app exits mid-game
• Player and host IDs now use UUID.randomUUID() instead of System.currentTimeMillis() to guarantee uniqueness

Related issue

Closes #54

Layer(s) affected

• UI (screens / components)
• Game Logic (session / round / scoring / state)
• Domain (model)
• Infrastructure (Firebase / audio / adapters)

Checklist

• Code compiles without errors
• Tested on emulator or device
• No unrelated changes included
• Updated README if needed