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>

src/activities/settings/SettingsActivity.cpp

@@ -20,7 +20,8 @@ constexpr int changeTabsMs = 700;
 constexpr int displaySettingsCount = 7;
 const SettingInfo displaySettings[displaySettingsCount] = {
     // Should match with SLEEP_SCREEN_MODE
-    SettingInfo::Enum("Sleep Screen", &CrossPointSettings::sleepScreen, {"Dark", "Light", "Custom", "Cover", "None"}),
+    SettingInfo::Enum("Sleep Screen", &CrossPointSettings::sleepScreen,
+                      {"Dark", "Light", "Custom", "Cover", "None", "Cover + Custom"}),
     SettingInfo::Enum("Sleep Screen Cover Mode", &CrossPointSettings::sleepScreenCoverMode, {"Fit", "Crop"}),
     SettingInfo::Enum("Sleep Screen Cover Filter", &CrossPointSettings::sleepScreenCoverFilter,
                       {"None", "Contrast", "Inverted"}),