refactor: replace BackedDataProvider with React Query cache lookup

The masquerade fallback data was maintained in a separate context
provider, duplicating what React Query already caches. Now when
masquerading fails, the normal user's data is read directly from the
query cache via queryClient.getQueryData(). This eliminates
BackedDataProvider and its associated useEffect sync.

Co-Authored-By: Claude <noreply@anthropic.com>

Author: arbrandes