cpal 0.15 uses ALSA as its Linux backend, which does not expose
PulseAudio/PipeWire monitor sources. The previous approach searched
for 'monitor' in cpal device names, which never matched on most
Linux systems, silently falling back to the microphone input.
This fix:
- Detects the correct monitor source via 'pactl get-default-sink'
and 'pactl list short sources'
- Sets PULSE_SOURCE env var before cpal initialization so PulseAudio's
ALSA plugin routes the default input through the monitor
- Preserves the original name-based search as fallback when pactl is
unavailable
- No new dependencies added
Tested on PipeWire 1.0.5 with Realtek ALC897 (HD-Audio Generic).
