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

src/ui/game_screen.cpp

@@ -2,6 +2,7 @@
 #include "ui/ui_colors.hpp"
 #include "rendering/vk_context.hpp"
 #include "core/application.hpp"
+#include "core/appearance_composer.hpp"
 #include "addons/addon_manager.hpp"
 #include "core/coordinates.hpp"
 #include "core/input.hpp"
@@ -631,7 +632,7 @@ void GameScreen::render(game::GameHandler& gameHandler) {
     if (inventoryScreen.consumeEquipmentDirty() || gameHandler.consumeOnlineEquipmentDirty()) {
         updateCharacterGeosets(gameHandler.getInventory());
         updateCharacterTextures(gameHandler.getInventory());
-        if (auto* ac = core::Application::getInstance().getAppearanceComposer()) ac->loadEquippedWeapons();
+        if (appearanceComposer_) appearanceComposer_->loadEquippedWeapons();
         inventoryScreen.markPreviewDirty();
         // Update renderer weapon type for animation selection
         auto* r = core::Application::getInstance().getRenderer();