Carry track duration through the queue pipeline

ApiTrackLike/QueueItem had no duration field, so toQueueItem(s) always
dropped it and toTrack() hardcoded a new track's duration to 0, relying
entirely on the <audio> element's durationchange/loadedmetadata events
to fill it in once that track actually became current. Upcoming queue
rows never reach that point, so "A continuación" always showed a blank
duration even though the backend already returns it.

Add duration to both types, thread it through toQueueItems/toQueueItem/
trackFromQueueItem, and seed toTrack() from it. The real-time audio
sync still overwrites it once a track actually plays, so this only
fills the gap for tracks that haven't played yet.

Also map MixItemApplicationResponse's durationSeconds through
mixItemTrack, since mixes use a differently-named field.

web-player/src/lib/data/mixes.ts

@@ -13,6 +13,7 @@ export function mixItemTrack(item: MixItem): ApiTrackLike {
 		id: item.trackId,
 		title: item.title,
 		artist: item.artist,
+		duration: item.durationSeconds,
 		listensCount: item.listensCount
 	};
 }