Support m3u playlist files to define chapter order

The chapter order on the tonie is already fully API-controlled via
SortChaptersOfTonie/PATCH. This adds an explicit way to define that
order via an .m3u/.m3u8 file placed in the tonie folder, instead of
relying on alphabetical filenames:

- internal/syncer/playlist.go: finds a single playlist file in the
  tonie folder, parses its entries (ignoring blanks/#-comments), and
  reorders local tracks accordingly. Tracks not referenced by the
  playlist are appended afterwards so nothing is silently dropped;
  playlist entries without a matching local file are ignored.
  Multiple playlist files in the same folder is an error (ambiguous
  order).
- ListLocalTracks now prefers this playlist-defined order, falling
  back to alphabetical filename order when no playlist is present.
- Tests covering playlist ordering, unreferenced-track handling,
  missing entries, and the multiple-playlists error case.
- README documents the .m3u/.m3u8 convention.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

internal/syncer/syncer.go

@@ -56,7 +56,12 @@ func TitleFromFilename(name string) string {
 	return title
 }
 
-// ListLocalTracks returns audio files in folder, sorted by filename (defines chapter order).
+// ListLocalTracks returns the audio files in folder that should be synced,
+// in chapter order.
+//
+// If the folder contains a single .m3u/.m3u8 playlist file, its entry order
+// defines the chapter order (see orderTracksByPlaylist); otherwise tracks
+// are sorted alphabetically by filename.
 func ListLocalTracks(folder string) ([]Track, error) {
 	entries, err := os.ReadDir(folder)
 	if err != nil {
@@ -78,6 +83,18 @@ func ListLocalTracks(folder string) ([]Track, error) {
 		})
 	}
 	sort.Slice(tracks, func(i, j int) bool { return tracks[i].Path < tracks[j].Path })
+
+	playlistPath, err := findPlaylistFile(folder, entries)
+	if err != nil {
+		return nil, err
+	}
+	if playlistPath != "" {
+		tracks, err = orderTracksByPlaylist(folder, playlistPath, tracks)
+		if err != nil {
+			return nil, err
+		}
+	}
+
 	return tracks, nil
 }