Introduces two reusable loading components to standardize loading states:
- Spinner: Base spinner with accessibility support (role, aria-label)
- LoadingState: Semantic loading indicator combining spinner and label
Updates loading patterns across the app:
- Full-screen/block loading now uses LoadingState (App, router, files, file)
- Inline button loading states show spinner + text with aria-busy
- Message list loading replaced with MessageSkeleton component
- Removes duplicate SpinnerIcon definition from PermissionFooter
Improves UX and accessibility:
- Loading screens centered and full-height
- Consistent ellipsis character (…)
- Semantic accessibility (role="status", aria-live, aria-busy)
- Adds CSS variables for banner styling
