fix(scanner, browse): fallback to original image when thumbnail generation fails for cover.jpg

When scanning image files (e.g. cover.jpg in audiobook folders),
thumbnailForImage can fail or produce a missing .thumbnails/cover.jpg.
This caused broken thumbnails in the grid.

- scanner.go buildThumbnailPath: after generating an image thumbnail,
  verify the output file exists via fs.Stat. If it does not exist,
  fall back to using the original image path as the thumbnail_path.
- browse.go GetThumbnail: if os.Stat(ThumbnailPath) fails and the
  media type is image, fall back to serving the original AbsPath.

All tests pass.

0 files changed, 0 insertions, 0 deletions