feat: Rework "Cover + Custom" sleep screens to show covers only when currently reading (#1256)
## Summary
The cover/custom setting was a bit misleading to me. Usually after you
start reading a book you never see your customs again unless a cover
fails to render for some reason. With this, you can easily show your
custom images by just sleeping from the menus instead of the reader
## Additional Context
* Add any other information that might be helpful for the reviewer
(e.g., performance implications, potential risks,
specific areas to focus on).
---
### AI Usage
While CrossPoint doesn't have restrictions on AI tools in contributing,
please be transparent about their usage as it
helps set the right context for reviewers.
Did you use AI tools to help write this code? _**< NO >**_
authored by