Non-URL input (e.g. !play classical music) searches YouTube for 10
results and picks one randomly. Also fixes --flat-playlist returning
"NA" as the URL for single videos by falling back to the original
input URL.
_resolve_title replaced with _resolve_tracks using --flat-playlist to
enumerate playlist entries. cmd_play enqueues each track individually,
with truncation when the queue is nearly full. Single-video behavior
unchanged.