refactor(web): playlistTrackToRef helper to dedupe TrackRef

reconstruction (#375)

PlaylistTrackRow.svelte and PlaylistCard.toTrackRefs both rebuilt
TrackRef from PlaylistTrack with the same field-by-field literal.
Server adding a new TrackRef field would have needed both sites
updated; now one helper owns the mapping.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

web/src/lib/playlists/playlistTrackToRef.ts

@@ -0,0 +1,22 @@
+// Converts a PlaylistTrack row into the minimal TrackRef shape used
+// by play queues + track menus. Centralizes the mapping so server
+// additions (new TrackRef fields) only update one site.
+//
+// Returns null when the upstream track has been removed (track_id
+// is null on the playlist row); callers should filter nulls.
+
+import type { PlaylistTrack, TrackRef } from '$lib/api/types';
+
+export function playlistTrackToRef(row: PlaylistTrack): TrackRef | null {
+  if (!row.track_id) return null;
+  return {
+    id: row.track_id,
+    title: row.title,
+    album_id: row.album_id ?? '',
+    album_title: row.album_title,
+    artist_id: row.artist_id ?? '',
+    artist_name: row.artist_name,
+    duration_sec: row.duration_sec,
+    stream_url: row.stream_url ?? ''
+  };
+}