fix: replay gain not applying to tracks

Replay gain was not working because track objects were created manually without
including replay gain metadata from the Subsonic API response.

Changes:
- Add songToTrack() helper function to properly map SubsonicSong to Track with
  replayGainTrackDb, replayGainAlbumDb, and replayGainPeak fields
- Add audio_update_replay_gain Tauri command for dynamic volume recalculation
  when replay gain settings change mid-playback
- Add updateReplayGainForCurrentTrack() to recalculate volume when toggling
  replay gain setting
- Fetch fresh track data on cold resume (app relaunch) to ensure replay gain
  values are current from server
- Update all files that create track objects to use songToTrack()

Fixes issue where toggling replay gain ON/OFF or changing between track/album
mode had no effect on currently playing or newly played tracks.

src-tauri/src/lib.rs

@@ -405,6 +405,7 @@ pub fn run() {
             audio::audio_stop,
             audio::audio_seek,
             audio::audio_set_volume,
+            audio::audio_update_replay_gain,
             audio::audio_set_eq,
             audio::audio_preload,
             audio::audio_set_crossfade,