Implements org.mpris.MediaPlayer2 and org.mpris.MediaPlayer2.Player
D-Bus interfaces, allowing desktop environment media keys and tools
like playerctl to control playback (play, pause, next, previous)
and display now-playing metadata.
Also adds _FallbackServiceInterface, _FallbackVariant and
_FallbackNameFlag so all tests pass without dbus-fast installed;
the module gracefully degrades when the library is unavailable.
- Fix dead exception handler in discord_rpc.py (unreachable DiscordNotFound)
- Replace QMediaPlayer=None stub with proper _StubQMediaPlayer in player.py
- Make AudioVisualizer color stops Qt-free (raw tuples instead of QColor)
- Add conftest QApplication creation check for headless CI detection
- Add test_gui_widgets.py _qt_available() guard for headless environments
- Add test_main.py (30 tests) for main.py entry point and env vars
- Add test_import_fallbacks.py (14 tests) for Qt import fallback paths
- Extend test_gui_helpers.py with _css, _get_thumbnail_nam, _cached_thumb_path tests
- Extend test_discord_rpc.py with direct _connect() test for DiscordNotFound
- Extend test_player_helpers.py with SearchWorker test-mode tests
- Ensure import fallback tests clean up sys.modules to avoid cross-test pollution
100% coverage achieved on: config.py, discord_rpc.py, main.py, music_db.py
