refactor: decouple Application singleton by extracting core subsystems and updating interfaces

- Add `audio::AudioCoordinator` interface and implementation
- Modify `Application` to reduce singleton usage and move controller responsibilities:
  - application.hpp
  - application.cpp
- Update UI and audio headers/sources:
  - game_screen.hpp
  - game_screen.cpp
  - ui_manager.hpp
  - audio_coordinator.hpp
  - audio_coordinator.cpp
- Project config touched:
  - CMakeLists.txt

CMakeLists.txt

@@ -544,6 +544,7 @@ set(WOWEE_SOURCES
 
     # Audio
     src/audio/audio_engine.cpp
+    src/audio/audio_coordinator.cpp
     src/audio/music_manager.cpp
     src/audio/footstep_manager.cpp
     src/audio/activity_sound_manager.cpp