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_header.py

@@ -6,7 +6,7 @@ import types
 import pytest
 from rich.cells import cell_len
 
-from somafm_tui.widgets.header import HeaderBar
+from beefy_player.widgets.header import HeaderBar
 
 TIME = re.compile(r"\d\d:\d\d:\d\d$")
 DATE = re.compile(r"[A-Z][a-z]{2} \d\d [A-Z][a-z]{2}$")