🔧 SHOW_CUSTOM_BOOTSCREEN sanity-check (#23807)

Marlin/src/inc/SanityCheck.h

@@ -860,6 +860,8 @@ static_assert(Y_MAX_LENGTH >= Y_BED_SIZE, "Movement bounds (Y_MIN_POS, Y_MAX_POS
  */
 #if ENABLED(SHOW_CUSTOM_BOOTSCREEN) && NONE(HAS_MARLINUI_U8GLIB, TOUCH_UI_FTDI_EVE)
   #error "SHOW_CUSTOM_BOOTSCREEN requires Graphical LCD or TOUCH_UI_FTDI_EVE."
+#elif ENABLED(SHOW_CUSTOM_BOOTSCREEN) && DISABLED(SHOW_BOOTSCREEN)
+  #error "SHOW_CUSTOM_BOOTSCREEN requires SHOW_BOOTSCREEN."
 #elif ENABLED(CUSTOM_STATUS_SCREEN_IMAGE) && !HAS_MARLINUI_U8GLIB
   #error "CUSTOM_STATUS_SCREEN_IMAGE requires a 128x64 DOGM B/W Graphical LCD."
 #endif