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

include/ui/ui_manager.hpp

@@ -13,7 +13,7 @@ union SDL_Event;
 namespace wowee {
 
 // Forward declarations
-namespace core { class Window; enum class AppState; }
+namespace core { class Window; class AppearanceComposer; enum class AppState; }
 namespace auth { class AuthHandler; }
 namespace game { class GameHandler; }
 
@@ -69,6 +69,11 @@ public:
     CharacterScreen& getCharacterScreen() { return *characterScreen; }
     GameScreen& getGameScreen() { return *gameScreen; }
 
+    // Dependency injection forwarding (Phase A singleton breaking)
+    void setAppearanceComposer(core::AppearanceComposer* ac) {
+        if (gameScreen) gameScreen->setAppearanceComposer(ac);
+    }
+
 private:
     core::Window* window = nullptr;