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.

package-lock.json

@@ -1,18 +1,17 @@
 {
   "name": "psysonic",
-  "version": "1.18.0",
+  "version": "1.21.0",
   "lockfileVersion": 3,
   "requires": true,
   "packages": {
     "": {
       "name": "psysonic",
-      "version": "1.18.0",
+      "version": "1.21.0",
       "dependencies": {
         "@tauri-apps/api": "^2",
         "@tauri-apps/plugin-dialog": "^2.6.0",
         "@tauri-apps/plugin-fs": "^2.4.5",
         "@tauri-apps/plugin-global-shortcut": "^2",
-        "@tauri-apps/plugin-notification": "^2",
         "@tauri-apps/plugin-shell": "^2",
         "@tauri-apps/plugin-store": "^2",
         "@tauri-apps/plugin-window-state": "^2.4.1",
@@ -1494,15 +1493,6 @@
         "@tauri-apps/api": "^2.8.0"
       }
     },
-    "node_modules/@tauri-apps/plugin-notification": {
-      "version": "2.3.3",
-      "resolved": "https://registry.npmjs.org/@tauri-apps/plugin-notification/-/plugin-notification-2.3.3.tgz",
-      "integrity": "sha512-Zw+ZH18RJb41G4NrfHgIuofJiymusqN+q8fGUIIV7vyCH+5sSn5coqRv/MWB9qETsUs97vmU045q7OyseCV3Qg==",
-      "license": "MIT OR Apache-2.0",
-      "dependencies": {
-        "@tauri-apps/api": "^2.8.0"
-      }
-    },
     "node_modules/@tauri-apps/plugin-shell": {
       "version": "2.3.5",
       "resolved": "https://registry.npmjs.org/@tauri-apps/plugin-shell/-/plugin-shell-2.3.5.tgz",