Visual mode: Don't show red background while loading edit capability #6667
Description
Parent issue: #6665
When the timeline expanded section is loading (checking whether a field can be updated via /api/can-update-sequence-props), the field shows a red background (notEditableBackground). This is visually jarring — we should either show no special background or a subtle loading state while the check is in progress, and only show the red background if the result confirms the field is not editable.