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.

settings.py

@@ -10,12 +10,15 @@ from mtsmusic import CONFIG_DIR
 
 SETTINGS_PATH = CONFIG_DIR / "settings.json"
 
+DEFAULT_DOWNLOAD_DIR = str(Path.home() / "Music" / "MTS Music")
+
 DEFAULTS: dict[str, Any] = {
     "volume": 80,            # громкость 0..100
     "seek_step": 5,          # шаг перемотки стрелками, секунды
     "autoplay": False,       # запускать воспроизведение сразу
     "start_queue": "wave",   # wave | favorites | history
     "wave_from_history": True,  # продолжать волну с последних треков
+    "download_dir": DEFAULT_DOWNLOAD_DIR,  # куда сохранять скачанные MP3
 }
 
 START_QUEUES = ("wave", "favorites", "history")
@@ -47,6 +50,11 @@ def _coerce(key: str, value: Any) -> Any:
     if key == "start_queue":
         text = str(value)
         return text if text in START_QUEUES else DEFAULTS[key]
+    if key == "download_dir":
+        text = str(value).strip()
+        if not text:
+            return DEFAULTS[key]
+        return str(Path(text).expanduser())
     return value