files: video previews are made on the Transcode device too

The setting's device makes every video preview: on the file's own machine
when it is there (the hub tells it by header), else on the device's machine,
which reads the file through the hub by a one-file token, and the preview is
kept in the file's machine's own cache (PUT /api/files/thumb, refused if the
file changed). Images and covers stay on the file's machine's CPU. If the
device's machine can't, the file's makes it as before.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/api/play.go

@@ -225,6 +225,9 @@ func (s *Server) playPiece(w http.ResponseWriter, r *http.Request) {
 // transcodeChoice is the setting; nil is the machine the file is on.
 func (s *Server) transcodeChoice() *store.Transcode {
 	var t *store.Transcode
+	if s.store == nil {
+		return nil
+	}
 	s.store.View(func(st *store.State) {
 		if st.Transcode != nil {
 			c := *st.Transcode