feat(offline): #427 S4a — Shuffle all + offline system-playlist gate

Headline of S4. "Shuffle all" is always present (home app-bar
shuffle icon); the pool degrades with reachability:
- online  → GET /api/library/shuffle?limit=N (new): N random
  library tracks server-side, per-user quarantine filtered
  (ListRandomTracksForUser, ORDER BY random()).
- offline → ShuffleSource shuffles the whole local cache index
  (audio_cache_index ∩ cached_tracks, names from cached_artists/
  albums) — a UNION over liked AND recently-played, since the
  two-bucket split is storage-only and never filters playback.

Offline gating: refreshable (singleton) system playlists need the
live build/shuffle endpoints, so their tile play is disabled when
offlineProvider is true — Shuffle all is the offline path. User
playlists still play from cache.

playlist_card now reads offlineProvider in build → wrapped the
direct-render widget test in ProviderScope (offlineProvider is
smoke-safe from S1: tracked timers, no connectivity mount).

S4b (offline Recently-played / Liked browsable surfaces over the
cache index) next.

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

internal/db/dbq/tracks.sql.go

@@ -255,6 +255,73 @@ func (q *Queries) ListArtistTracksForUser(ctx context.Context, arg ListArtistTra
 	return items, nil
 }
 
+const listRandomTracksForUser = `-- name: ListRandomTracksForUser :many
+SELECT t.id, t.title, t.album_id, t.artist_id, t.track_number, t.disc_number, t.duration_ms, t.file_path, t.file_size, t.file_format, t.bitrate, t.mbid, t.genre, t.added_at, t.updated_at,
+       albums.title AS album_title,
+       artists.name AS artist_name
+FROM tracks t
+JOIN albums  ON albums.id  = t.album_id
+JOIN artists ON artists.id = t.artist_id
+WHERE NOT EXISTS (
+      SELECT 1 FROM lidarr_quarantine q
+      WHERE q.user_id = $1 AND q.track_id = t.id
+  )
+ORDER BY random()
+LIMIT $2
+`
+
+type ListRandomTracksForUserParams struct {
+	UserID pgtype.UUID
+	Limit  int32
+}
+
+type ListRandomTracksForUserRow struct {
+	Track      Track
+	AlbumTitle string
+	ArtistName string
+}
+
+// #427 S4: backing query for GET /api/library/shuffle — the online
+// source for "Shuffle all". N random tracks across the whole
+// library, per-user-quarantine filtered. $1 user_id, $2 limit.
+func (q *Queries) ListRandomTracksForUser(ctx context.Context, arg ListRandomTracksForUserParams) ([]ListRandomTracksForUserRow, error) {
+	rows, err := q.db.Query(ctx, listRandomTracksForUser, arg.UserID, arg.Limit)
+	if err != nil {
+		return nil, err
+	}
+	defer rows.Close()
+	var items []ListRandomTracksForUserRow
+	for rows.Next() {
+		var i ListRandomTracksForUserRow
+		if err := rows.Scan(
+			&i.Track.ID,
+			&i.Track.Title,
+			&i.Track.AlbumID,
+			&i.Track.ArtistID,
+			&i.Track.TrackNumber,
+			&i.Track.DiscNumber,
+			&i.Track.DurationMs,
+			&i.Track.FilePath,
+			&i.Track.FileSize,
+			&i.Track.FileFormat,
+			&i.Track.Bitrate,
+			&i.Track.Mbid,
+			&i.Track.Genre,
+			&i.Track.AddedAt,
+			&i.Track.UpdatedAt,
+			&i.AlbumTitle,
+			&i.ArtistName,
+		); err != nil {
+			return nil, err
+		}
+		items = append(items, i)
+	}
+	if err := rows.Err(); err != nil {
+		return nil, err
+	}
+	return items, nil
+}
+
 const listTracksByAlbum = `-- name: ListTracksByAlbum :many
 SELECT id, title, album_id, artist_id, track_number, disc_number, duration_ms, file_path, file_size, file_format, bitrate, mbid, genre, added_at, updated_at FROM tracks
 WHERE album_id = $1