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.

1 files changed, 10 insertions, 1 deletions

diff --git a/internal/scanner/scanner.go b/internal/scanner/scanner.go
index 30b20d1..34e0ca5 100644
--- a/internal/scanner/scanner.go
+++ b/internal/scanner/scanner.go
@@ -202,7 +202,16 @@ func (s *FSScanner) buildThumbnailPath(ctx context.Context, path, setPath string
 		if ext == ".svg" {
 			return path, nil
 		}
-		return s.thumbnailForImage(ctx, path, setPath)
+		thumbPath, err := s.thumbnailForImage(ctx, path, setPath)
+		if err != nil {
+			return "", err
+		}
+		if thumbPath != "" {
+			if _, statErr := s.fs.Stat(thumbPath); statErr == nil {
+				return thumbPath, nil
+			}
+		}
+		return path, nil
 	}
 	return "", nil
 }