feat(providers): add a pluggable station-catalogue seam

The app now holds one RadioProvider and asks it for stations, song history and stream URLs instead of importing SomaFM's HTTP client directly.

- models.py: provider-neutral Song/Station plus the shared parsers every catalogue needs (Station.provider/.uid keep ids unique once there is more than one).
- providers/base.py: the RadioProvider interface and the toolkit it needs in practice - httpx client factory, .pls/.m3u resolution with an 8h disk cache, and a default stream_candidates() strategy.
- providers/somafm/: everything SomaFM-specific (channels.json, songs/<id>.json, PLS layout, guessed icecast mirrors) behind SomaFMProvider.
- providers/__init__.py: registry, get_provider() and --provider, remembered in the config file.

Widgets render branding, splash copy and the help credits from the active provider, so a second catalogue needs no UI changes; with SomaFM selected the screen is byte-identical to before.

tests/test_main.py

@@ -1,6 +1,8 @@
 from __future__ import annotations
 
-from somafm_tui.__main__ import _build_parser, _list_stations
+import pytest
+
+from beefy_player.__main__ import _build_parser, _list_stations
 
 
 def test_parser_defaults_and_flags() -> None:
@@ -19,6 +21,14 @@ def test_parser_defaults_and_flags() -> None:
     assert args.autoplay is True
     assert args.no_mpris is True
     assert args.spectrum_source == "monitor"
+    assert args.provider is None  # the config file decides
+
+
+def test_provider_flag_is_validated() -> None:
+    parser = _build_parser()
+    assert parser.parse_args(["--provider", "somafm"]).provider == "somafm"
+    with pytest.raises(SystemExit):
+        parser.parse_args(["--provider", "no-such-catalogue"])
 
 
 def test_list_stations_offline(capsys) -> None: