JACK routes through the JACK graph (resamples to the graph rate), so it
can't be bit-perfect — but nothing told the user. Add a `backend-is-jack`
flag (settings.rs computes it beside alsa/pipewire at both set-sites;
state.slint property) and a WarningBanner in AudioSettings, gated on it,
pointing at ALSA-direct / PipeWire-passthrough for bit-perfect. UI-only;
backend behavior unchanged.
Blind change (no build this session) — validates on the next qbz_ui build.
cc
vicrodh
6f7ff9956c