merge: PR #9 — Replay Gain fix (trbn1) + conflict resolution

Merges songToTrack() helper and audio_update_replay_gain Tauri command
from @trbn1 without losing v1.22.0 DnD/queue-management work:

- All track construction sites use songToTrack() for correct replay gain
- psy-drag system (mouse-event DnD) preserved across all components
- enqueueAt() position-aware insertion preserved in QueuePanel
- updatePlaylist / smart-save / active-playlist tracking preserved

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

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);

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,