refactor!: rename somafm-tui to beefy-player

The app is now a general internet-radio player rather than a SomaFM client, so the package, console script, MPRIS identity, mpv namespaces, macOS tap names, XDG directories and docs all move to beefy-player.

Environment variables are now BEEFY_* (the old SOMAFM_* spellings are still honoured) and an existing ~/.config/somafm-tui/config.json is migrated on first run, so favourites and volume survive the rename.

The visible interface is untouched: same layout, same logo, same keys.

tests/test_mediasession.py

@@ -3,8 +3,8 @@ from __future__ import annotations
 import queue
 import sys
 
-from somafm_tui import mediasession
-from somafm_tui.mediasession import (
+from beefy_player import mediasession
+from beefy_player.mediasession import (
     MediaSession,
     NowPlayingSession,
     create_media_session,
@@ -12,7 +12,7 @@ from somafm_tui.mediasession import (
 
 
 def test_linux_uses_mpris() -> None:
-    from somafm_tui.mpris import MprisService
+    from beefy_player.mpris import MprisService
 
     session = create_media_session(queue.Queue(), enabled=False, platform="linux")
     assert isinstance(session, MprisService)