Fix non-square trickplay thumbnail rendering and seek buffering overlay.

[VX-101]

Summary

Fixes trickplay thumbnail rendering for non-square tile images and improves the seek buffering transition.

• Calculate the thumbnail row from TileWidth
• Avoid preloading a tile image past the last valid trickplay tile
• Recreate trickplay Poster nodes without dropping the current preview update
• Show the buffering overlay when a seek is committed and while playback is buffering

Details

Jellyfin reports TileWidth as the number of thumbnails per row and TileHeight as the number of thumbnails per column. The Roku client was using TileHeight when calculating the thumbnail row inside a trickplay tile image. That works for square layouts like 10x10, but is wrong for non-square layouts.

The preload calculation could also request one tile image beyond the valid range when the thumbnail count exactly filled a tile image.

The preview node update now continues after creating or recreating a Poster node, so the current preview update still applies URI, size, clipping, translation, and visibility.

The seek path now starts the existing buffering overlay before applying the seek. The buffering state handler also starts the overlay so the player does not show a blank screen while playback resumes.

Testing

• Ran npm run build successfully.
• Sideloaded the generated Roku package
• Tested manual seek and trickplay previews on Roku
• Tested playback transitions that enter buffering
• Created a local 5x10 trickplay test clip with visible thumbnail markers
• Verified the old client showed incorrect thumbnail rows with 5x10 trickplay images
• Verified the fixed client uses the correct thumbnail row for non-square trickplay tile images

[VX-101]

Extra testing was performed with a local 5x10 trickplay test clip containing visible thumbnail markers. The old client displayed incorrect thumbnail rows for non-square trickplay tile images, while the updated client selected the correct thumbnails. This confirms the fix handles non-square trickplay layouts correctly.

Test file https://drive.google.com/drive/folders/1pFmQgKSTjza6vTE6vyEZYNWXZRBQROe7?usp=drive_link