feat(web): a playlist entry whose file is missing greys out and is skipped — #2527

The row treatment for a dead playlist entry already existed -- muted
text, no play on click, no drag, no kebab, never "now playing" -- but it
only fired for track_id === null, the track-deleted case. A missing file
kept a live-looking row that failed on click.

The behavioural gate now covers both, and the presentation distinguishes
them, because they mean different things to the person reading the list.
A removed track is gone for good and keeps the strikethrough. A missing
file is a track we still have -- history, likes, the lot -- whose bytes
aren't on disk right now, so it gets an explicit "File missing" and a
title explaining it stays in the playlist and comes back on its own if
the file does. A strikethrough there would claim it was deleted, which
is a lie about a file the scanner may well adopt back tomorrow (#2528).

Skipping routes through playlistTrackToRef, which already returned null
for removed tracks and whose callers already filter nulls. Adding the
unavailable check there means every queue builder -- PlaylistCard,
systemRefetch, the detail page -- skips a missing file without any of
them learning what missing_since is.

Remove stays available on a dead row: the owner must still be able to
take it out of their own list.

web/src/lib/playlists/playlistTrackToRef.ts

@@ -2,13 +2,19 @@
 // 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.
+// Returns null when the row cannot be played, for either of the two
+// reasons a playlist entry can outlive its track:
+//   - track_id is null — the track was removed from the library and only
+//     the playlist's text snapshot remains.
+//   - unavailable — the track is still there, with its history intact,
+//     but its file is missing from disk (#2527).
+// Callers filter nulls, which is what makes a missing file "get skipped"
+// everywhere a queue is built rather than at each call site.
 
 import type { PlaylistTrack, TrackRef } from '$lib/api/types';
 
 export function playlistTrackToRef(row: PlaylistTrack): TrackRef | null {
-  if (!row.track_id) return null;
+  if (!row.track_id || row.unavailable) return null;
   return {
     id: row.track_id,
     title: row.title,