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.

---

### 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 >**_

---------

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

src/CrossPointSettings.h

@@ -15,7 +15,15 @@ class CrossPointSettings {
   CrossPointSettings(const CrossPointSettings&) = delete;
   CrossPointSettings& operator=(const CrossPointSettings&) = delete;
 
-  enum SLEEP_SCREEN_MODE { DARK = 0, LIGHT = 1, CUSTOM = 2, COVER = 3, BLANK = 4, SLEEP_SCREEN_MODE_COUNT };
+  enum SLEEP_SCREEN_MODE {
+    DARK = 0,
+    LIGHT = 1,
+    CUSTOM = 2,
+    COVER = 3,
+    BLANK = 4,
+    COVER_CUSTOM = 5,
+    SLEEP_SCREEN_MODE_COUNT
+  };
   enum SLEEP_SCREEN_COVER_MODE { FIT = 0, CROP = 1, SLEEP_SCREEN_COVER_MODE_COUNT };
   enum SLEEP_SCREEN_COVER_FILTER {
     NO_FILTER = 0,