Feat: Zen Sleep Timer with Fade Out

### 🚀 The feature

I propose implementing a Zen Sleep Timer to enhance the bedtime listening experience.

Key features should include:

* Flexible Duration: A slider allowing users to set a timer from 1 minute up to 8 hours.
* Fade-Out Effect: The audio volume should smoothly fade out over the last 60 seconds of the timer to prevent a jarring silence when it stops.
* End of Chapter Option: A quick option to set the timer to the remaining duration of the current chapter.
* Easy Access: A dedicated button (moon icon) on the player screen for quick access.

### Motivation, pitch

Many users enjoy listening to audiobooks or stories to fall asleep. However, without a sleep timer, the audio continues playing all night, which can:

* Drain the device's battery.
* Cause users to lose their spot in the story.