feat(settings): add "Cover + Custom" sleep screen mode (#582)

## Summary

Allows to fallback to custom sleep screens if the book does not have a
cover.

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### 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? _**< partially >**_

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Co-authored-by: mrtnvgr <root@unixis.fun>

USER_GUIDE.md

@@ -103,6 +103,7 @@ The Settings screen allows you to configure the device's behavior. There are a f
   - "Custom" - Custom images from the SD card; see [Sleep Screen](#36-sleep-screen) below for more information
   - "Cover" - The book cover image (Note: this is experimental and may not work as expected)
   - "None" - A blank screen
+  - "Cover + Custom" - The book cover image, fallbacks to "Custom" behavior
 - **Sleep Screen Cover Mode**: How to display the book cover when "Cover" sleep screen is selected:
   - "Fit" (default) - Scale the image down to fit centered on the screen, padding with white borders as necessary
   - "Crop" - Scale the image down and crop as necessary to try to to fill the screen (Note: this is experimental and may not work as expected)