Smooth login music start and lower auth-screen volume

Add configurable fade-in support to MusicManager playback paths and use it for auth/login intro tracks to avoid abrupt starts. Apply a login-only 20% music attenuation while the auth screen is active, then restore the previous music volume when leaving login so in-game volume remains unchanged.

include/ui/auth_screen.hpp

@@ -109,6 +109,8 @@ private:
 
     bool musicInitAttempted = false;
     bool musicPlaying = false;
+    bool loginMusicVolumeAdjusted_ = false;
+    int savedMusicVolume_ = 30;
 };
 
 }} // namespace wowee::ui