Expand bit depth and FLAC codec support

[maximmaxim345]

WIP to test Sendspin/aiosendspin#129

Requires aiosendspin from that PR branch to be installed manually.

Adds multi-bit-depth audio output (16/24-bit) and FLAC codec decoding. Format detection probes the audio device on startup to advertise supported formats to the server.

Changes:

• detect_supported_audio_formats() probes device for supported rates/depths/channels
• New FlacDecoder class decodes FLAC frames using PyAV
• Volume scaling handles 16/24/32-bit samples with dedicated code paths
• UI shows current audio format in status line

TODOs

• Simplify software volume handling by always using 32-bit internally, then convert to output bit depth at final output (maybe we should always use the highest bit depth since we do sw volume)
• Test on low-powered hardware (Pi Zero) to verify 32-bit internal processing doesn't cause performance issues
• Format detection takes ~200ms (6 probes). Could cache results or run async to reduce startup delay.