- online.rs: on Linux, advertise only WebView-decodable codecs (h264 video;
aac/mp3/opus/vorbis/flac audio) in PlaybackInfo so Jellyfin transcodes
HEVC/AV1/VP9/etc. to h264 HLS for the WebKitGTK <video> element.
- player: on Linux, don't load video into MPV (no embedded window — it would
start a redundant decode the frontend immediately stops). Add
PlayerController::set_current_item to keep queue/UI/remote-transfer state in
sync without loading the item into the playback backend.