Removed the home-grown backend probing / priority logic (pipewire → pulse → alsa) when no device is explicitly configured.
Kept support for explicitly specifying an audio_output_device in the config/CLI; if provided, it is passed through to mpv.
Added debug logging after mpv initialization to show:
current ao
current audio-device
the full audio-device-list
Selects the best available audio output backend for mpv.
The selection follows a clear priority:
1. A fully qualified device string (e.g., 'alsa/plughw:...') is used directly.
2. A simple device name tries PipeWire, then PulseAudio, then ALSA.
3. No device specified (or 'default') tries the default for PipeWire, PulseAudio, and ALSA in order.
Updates the MpvMediaPlayer constructor (__init__) to accept an initial_volume parameter.
This allows the volume to be set immediately when the mpv process is created, applying the saved setting at startup.