[Feature] Add Emby BIF preview thumbnails while scrubbing

[MikaelKW]

What problem does this solve?

When Moonfin is connected to an Emby server, preview thumbnails do not appear
while scrubbing or seeking through the playback timeline.

The same media shows timeline preview thumbnails in Emby's native clients,
provided that Emby has generated the preview images.

Moonfin already supports scrub-preview thumbnails with Jellyfin trickplay
images, so adding support for Emby's preview format would provide more
consistent playback functionality between Jellyfin and Emby connections.

Proposed solution

Add support for retrieving and displaying Emby's BIF preview thumbnails while
scrubbing the playback timeline.

Emby exposes these previews through its BIF endpoint: /Videos/{ItemId}/index.bif

Moonfin could parse the BIF file and use the contained frames with its existing
timeline preview interface.

The feature should fail gracefully and simply show no preview when the Emby
server has not generated a BIF file for the selected media.

Platform

All / Not platform-specific

Alternatives considered

No response

Additional context

Moonfin appears to already distinguish Emby BIF trickplay support through
supportsBifTrickplay, but I could not find an implementation that retrieves
or parses Emby's index.bif files.

The existing Moonfin player already has a scrub-preview interface for Jellyfin
trickplay images, so hopefully the Emby BIF frames could be connected to that
existing UI.

Please let me know if I can help with testing, API responses, or BIF files generated by my Emby server.