[ flox1an/nostube ] fix: video server availability HEAD request loop
Fixed continuous HEAD requests to Blossom servers when checking video
availability. The caching mechanism was broken because setting status
to 'checking' would overwrite the `lastChecked` timestamp, causing the
cache check to fail on subsequent calls.
Changes:
- Preserve `lastChecked` when transitioning to 'checking' status
- Use stable empty array constant for `videoUrls` to prevent unnecessary