Add support for fading out on sleep timer end

Playback will start to fadeout during last 60 seconds of the sleep timer. Once faded out, playback will be paused, volume reset, and playback seeked to start of fadeout.

ios/App/Shared/models/DeviceSettings.swift

@@ -19,6 +19,7 @@ class DeviceSettings: Object {
     @Persisted var languageCode: String = "en-us"
     @Persisted var downloadUsingCellular: String = "ALWAYS"
     @Persisted var streamingUsingCellular: String = "ALWAYS"
+    @Persisted var disableSleepTimerFadeOut: Bool = false
 }
 
 func getDefaultDeviceSettings() -> DeviceSettings {
@@ -36,6 +37,7 @@ func deviceSettingsToJSON(settings: DeviceSettings) -> Dictionary<String, Any> {
         "hapticFeedback": settings.hapticFeedback,
         "languageCode": settings.languageCode,
         "downloadUsingCellular": settings.downloadUsingCellular,
-        "streamingUsingCellular": settings.streamingUsingCellular
+        "streamingUsingCellular": settings.streamingUsingCellular,
+        "disableSleepTimerFadeOut": settings.disableSleepTimerFadeOut
     ]
 }