Adds a bit_perfect_mode field to SharedState and PlaybackEvent so the
frontend can display whether the active stream is:
- DirectHardware — ALSA hw: direct access (bit-perfect guaranteed)
- PluginFallback — ALSA plughw: with software resample (bit-perfect
lost; happens when the DAC doesnt support the stream's sample
rate, e.g., the Vanatoo case in #288)
- Disabled — shared system path (pipewire/pulse/CPAL)
try_init_stream_with_backend now takes the SharedState so it can
record the mode the moment the stream is successfully opened,
whether via ALSA direct or the CPAL/Rodio fallback.
cc