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/audio.rs

@@ -1623,6 +1623,22 @@ pub fn audio_set_volume(volume: f32, state: State<'_, AudioEngine>) {
     }
 }
 
+#[tauri::command]
+pub fn audio_update_replay_gain(
+    volume: f32,
+    replay_gain_db: Option<f32>,
+    replay_gain_peak: Option<f32>,
+    state: State<'_, AudioEngine>,
+) {
+    let (gain_linear, effective) = compute_gain(replay_gain_db, replay_gain_peak, volume);
+    let mut cur = state.current.lock().unwrap();
+    cur.replay_gain_linear = gain_linear;
+    cur.base_volume = volume.clamp(0.0, 1.0);
+    if let Some(sink) = &cur.sink {
+        sink.set_volume(effective);
+    }
+}
+
 #[tauri::command]
 pub fn audio_set_eq(gains: [f32; 10], enabled: bool, state: State<'_, AudioEngine>) {
     state.eq_enabled.store(enabled, Ordering::Relaxed);