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.
imonlinux