Add blank sleep screen option (#242)

## Summary

Very small change to add a blank ("None") sleep screen option, for those
who prefer a clean aesthetic.

Tested on X4 device.

src/activities/boot_sleep/SleepActivity.cpp

@@ -28,6 +28,10 @@ void SleepActivity::onEnter() {
   Activity::onEnter();
   renderPopup("Entering Sleep...");
 
+  if (SETTINGS.sleepScreen == CrossPointSettings::SLEEP_SCREEN_MODE::BLANK) {
+    return renderBlankSleepScreen();
+  }
+
   if (SETTINGS.sleepScreen == CrossPointSettings::SLEEP_SCREEN_MODE::CUSTOM) {
     return renderCustomSleepScreen();
   }
@@ -234,3 +238,8 @@ void SleepActivity::renderCoverSleepScreen() const {
 
   renderDefaultSleepScreen();
 }
+
+void SleepActivity::renderBlankSleepScreen() const {
+  renderer.clearScreen();
+  renderer.displayBuffer(EInkDisplay::HALF_REFRESH);
+}

src/activities/boot_sleep/SleepActivity.h

@@ -15,4 +15,5 @@ class SleepActivity final : public Activity {
   void renderCustomSleepScreen() const;
   void renderCoverSleepScreen() const;
   void renderBitmapSleepScreen(const Bitmap& bitmap) const;
+  void renderBlankSleepScreen() const;
 };

src/activities/settings/SettingsActivity.cpp

@@ -12,7 +12,7 @@ namespace {
 constexpr int settingsCount = 14;
 const SettingInfo settingsList[settingsCount] = {
     // Should match with SLEEP_SCREEN_MODE
-    {"Sleep Screen", SettingType::ENUM, &CrossPointSettings::sleepScreen, {"Dark", "Light", "Custom", "Cover"}},
+    {"Sleep Screen", SettingType::ENUM, &CrossPointSettings::sleepScreen, {"Dark", "Light", "Custom", "Cover", "None"}},
     {"Status Bar", SettingType::ENUM, &CrossPointSettings::statusBar, {"None", "No Progress", "Full"}},
     {"Extra Paragraph Spacing", SettingType::TOGGLE, &CrossPointSettings::extraParagraphSpacing, {}},
     {"Short Power Button Click", SettingType::TOGGLE, &CrossPointSettings::shortPwrBtn, {}},