YouTube's CDN advertises maxres/sddefault thumbnails for every video but
only generates them for sufficiently high-res uploads, so older/low-res
videos return 404 for those qualities. The view picked the best quality
and showed a blank placeholder on failure with no fallback.
VideoThumbnailView now accepts an ordered fallback chain and advances to
the next candidate via NukeUI's onCompletion when a load fails, so a valid
thumbnail is always shown. DeArrowVideoThumbnail feeds it DeArrow branding
first, then the video's own thumbnails best-quality-first.
