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/player/AudioPlayerSleepTimer.swift

@@ -125,7 +125,11 @@ extension AudioPlayer {
         if var sleepTimeRemaining = self.sleepTimeRemaining {
             sleepTimeRemaining -= 1
             self.sleepTimeRemaining = sleepTimeRemaining
-            
+          
+            if sleepTimeRemaining == 60 && self.isSleepTimerFadeOutEnabled() {
+                self.startFadeOut()
+            }
+          
             // Handle the sleep if the timer has expired
             if sleepTimeRemaining <= 0 {
                 self.handleSleepEnd()
@@ -154,5 +158,9 @@ extension AudioPlayer {
     private func isChapterSleepTimerSet() -> Bool {
         return self.sleepTimeChapterStopAt != nil
     }
-    
+  
+    private func isSleepTimerFadeOutEnabled() -> Bool {
+      let deviceSettings = Database.shared.getDeviceSettings()
+      return !deviceSettings.disableSleepTimerFadeOut
+    }
 }