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

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Did you use AI tools to help write this code? _**< NO >**_

src/activities/boot_sleep/SleepActivity.cpp

@@ -24,8 +24,13 @@ void SleepActivity::onEnter() {
     case (CrossPointSettings::SLEEP_SCREEN_MODE::CUSTOM):
       return renderCustomSleepScreen();
     case (CrossPointSettings::SLEEP_SCREEN_MODE::COVER):
-    case (CrossPointSettings::SLEEP_SCREEN_MODE::COVER_CUSTOM):
       return renderCoverSleepScreen();
+    case (CrossPointSettings::SLEEP_SCREEN_MODE::COVER_CUSTOM):
+      if (APP_STATE.lastSleepFromReader) {
+        return renderCoverSleepScreen();
+      } else {
+        return renderCustomSleepScreen();
+      }
     default:
       return renderDefaultSleepScreen();
   }