feat: download tracks with full ID3 metadata

- Add downloads.py: "Artist - Title.mp3" naming, no-overwrite dedupe,
  configurable download_dir, progress and per-item callbacks.
- Save maximum metadata via tags.py (ID3v2.3, mutagen): artists, album
  and album artist, track/disc, year/date, raw genre, label, duration,
  explicit flag, cover 600x600, plain and synced lyrics (MTS then
  LRCLIB), track URLs and Yandex/MTS ids.
- TUI/Qt: download current track and queue from the menu, "d" key in
  lists, folder picker in settings; CLI download gains -d/--dir and
  --no-tags.
- Add mutagen to requirements and document download/metadata in README.
- Fix MPRIS update_metadata crashing without dbus-next.

mpris.py

@@ -317,6 +317,8 @@ class MprisService:
         duration_ms: int = 0,
         art_url: str = "",
     ):
+        if not HAVE_DBUS:
+            return
         metadata: dict[str, Any] = {
             "mpris:trackid": Variant("o", track_object_path(track_id)),
             "mpris:length": Variant("x", int(duration_ms) * 1000),