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/routes/playlists/[id]/playlist.test.ts

@@ -52,8 +52,8 @@ const ownDetail: PlaylistDetail = {
   description: '', is_public: false, kind: 'user', system_variant: null, refreshable: false, seed_artist_id: null, cover_url: '', track_count: 2, duration_sec: 274,
   created_at: '', updated_at: '',
   tracks: [
-    { position: 0, track_id: 't1', album_id: 'a1', artist_id: 'ar1', title: 'A', artist_name: 'X', album_title: 'Y', duration_sec: 137, stream_url: '/api/tracks/t1/stream', added_at: '' },
-    { position: 1, track_id: 't2', album_id: 'a1', artist_id: 'ar1', title: 'B', artist_name: 'X', album_title: 'Y', duration_sec: 137, stream_url: '/api/tracks/t2/stream', added_at: '' }
+    { position: 0, track_id: 't1', album_id: 'a1', artist_id: 'ar1', title: 'A', artist_name: 'X', album_title: 'Y', duration_sec: 137, stream_url: '/api/tracks/t1/stream', added_at: '', unavailable: false },
+    { position: 1, track_id: 't2', album_id: 'a1', artist_id: 'ar1', title: 'B', artist_name: 'X', album_title: 'Y', duration_sec: 137, stream_url: '/api/tracks/t2/stream', added_at: '', unavailable: false }
   ]
 };