Tags::write built the on-disk path by concatenating mpd_music_dir with
the raw MPD song URI without filtering '..' segments, and
EditLibraryAlbum did the same with a direct TagLib::FileRef.
EditDirectoryName joined MPD-supplied directory paths with mpd_music_dir
the same way. A non-stream song whose URI contains '..' (served by a
remote or compromised MPD server) therefore resolved outside
mpd_music_dir: f.save() rewrote tags on an existing audio file at an
attacker-chosen path and, when the song was renamed,
boost::filesystem::rename moved the file.

The same mpd_music_dir + getURI pattern also appeared in read sinks:
TagsLyricsFetcher, the song info screen, and the tiny tag editor all
opened an arbitrary existing audio file at a traversed path with
TagLib::FileRef.

Add a shared normalizePath helper that collapses '.' and '..' segments
(relative paths cannot escape above their root, absolute paths keep
their leading slash) and apply it to every MPD-supplied URI/directory
before joining with mpd_music_dir. Legitimate relative URIs and
absolute local paths are unchanged; only traversal segments are removed.