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/components/PlaylistTrackRow.svelte

@@ -3,7 +3,8 @@
   import { draggable, type DragEventData } from '@neodrag/svelte';
   import LikeButton from './LikeButton.svelte';
   import TrackMenu from './TrackMenu.svelte';
-  import type { PlaylistTrack, TrackRef } from '$lib/api/types';
+  import type { PlaylistTrack } from '$lib/api/types';
+  import { playlistTrackToRef } from '$lib/playlists/playlistTrackToRef';
   import { offsetToDelta } from './queue-row-math';
 
   let {
@@ -22,22 +23,9 @@
 
   const isUnavailable = $derived(row.track_id === null);
 
-  // Construct a minimal TrackRef for the kebab menu when the upstream
-  // track still exists. When unavailable, the menu is hidden.
-  const liveTrack = $derived<TrackRef | null>(
-    row.track_id
-      ? ({
-          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 ?? ''
-        } as TrackRef)
-      : null
-  );
+  // Reconstruct a minimal TrackRef for the kebab menu when the
+  // upstream track still exists. When unavailable, the menu is hidden.
+  const liveTrack = $derived(playlistTrackToRef(row));
 
   let measuredRowHeight = 56; // sensible default; replaced on dragStart