feat(audio): add day/night-based BGM switching

include/Cubed/audio/audio_engine.hpp

@@ -26,6 +26,7 @@ public:
 
     void init();
     void play_bgm();
+    void change_bgm(const std::string& sound);
     void play_3d(const std::string& sound, const glm::vec3& pos,
                  bool check = false);
     void play_2d(const std::string& sound, bool check = false);

include/Cubed/audio/audio_fade.hpp

@@ -6,6 +6,7 @@ public:
     AudioFade(AudioSource* source, float fade_in = 0.0f, float fade_out = 0.0f);
     ~AudioFade();
     void update();
+    void reset();
 
 private:
     AudioSource* m_source = nullptr;