Replace PulseAudio monitor capture with direct file streaming.
Qt6's FFmpeg/PipeWire-native audio backend ignores PULSE_SINK so
the monitor captured silence. StreamingWorker now receives the
playing file path via a command queue and pipes it through ffmpeg
-re (real-time rate) directly to the server as audio_chunk events.
- Add switch_file() / stop_file() to StreamingWorker
- Replace ffmpeg pulse capture loop with file-based cmd loop
- DeckWidget.play() calls switch_file(path, position_ms)
- DeckWidget.pause()/stop() calls stop_file()
- Add now_playing socket relay in server.py
- listener.js handles now_playing event to show track title
- Add deck_glow emission from Qt deck play/pause/stop
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