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.

mtsmusic.py

@@ -872,34 +872,33 @@ def cmd_play(args: argparse.Namespace) -> None:
 
 
 def cmd_download(args: argparse.Namespace) -> None:
-    api = Api()
-    print_track_info(api, args.track_id)
-    url, _ = resolve_stream(api, args.track_id)
-    force_ipv4()
-    output = Path(args.output or f"{args.track_id}.mp3")
+    from downloads import download_track, stream_to_file, tag_downloaded
+    from settings import get_setting
 
-    with requests.get(
-        url,
-        stream=True,
-        headers={"User-Agent": USER_AGENT},
-        timeout=60,
-    ) as response:
-        response.raise_for_status()
-        total = int(response.headers.get("Content-Length") or 0)
-        done = 0
-        with output.open("wb") as handle:
-            for chunk in response.iter_content(chunk_size=256 * 1024):
-                if not chunk:
-                    continue
-                handle.write(chunk)
-                done += len(chunk)
-                if total:
-                    percent = done * 100 // total
-                    kib = done // 1024
-                    print(
-                        f"\r{percent:3d}%  {kib} KiB",
-                        end="",
-                        file=sys.stderr)
+    api = Api()
+    track = get_track(api, args.track_id)
+    print(fmt_track(track), file=sys.stderr)
+
+    def progress(done: int, total: int) -> None:
+        if total:
+            percent = done * 100 // total
+            print(
+                f"\r{percent:3d}%  {done // 1024} KiB",
+                end="",
+                file=sys.stderr)
+
+    with_tags = not args.no_tags
+    if args.output:
+        # Явный путь: скачиваем ровно в указанный файл, как раньше.
+        url, _ = resolve_stream(api, args.track_id)
+        output = stream_to_file(url, Path(args.output), progress)
+        if with_tags:
+            tag_downloaded(api, track, output)
+    else:
+        dest = args.dir or get_setting("download_dir")
+        output = download_track(api, track, dest, progress, with_tags)
+
+    if sys.stderr.isatty():
         print(file=sys.stderr)
     print(f"Сохранено: {output} ({output.stat().st_size} байт)")
 
@@ -1145,6 +1144,16 @@ def build_parser() -> argparse.ArgumentParser:
     download = sub.add_parser("download", help="скачать трек в MP3")
     download.add_argument("track_id")
     download.add_argument("-o", "--output", help="имя файла")
+    download.add_argument(
+        "-d",
+        "--dir",
+        help="папка для сохранения (по умолчанию из download_dir)",
+    )
+    download.add_argument(
+        "--no-tags",
+        action="store_true",
+        help="не записывать ID3-теги (текст, обложка, метаданные)",
+    )
     download.set_defaults(func=cmd_download)
 
     return parser