feat(audio): surface "JACK is not bit-perfect" in Audio settings

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

crates/qbz-ui/ui/settings/AudioSettings.slint

@@ -10,6 +10,7 @@ import { SettingsState, DacWizardActions } from "../state.slint";
 import { QbzToggle } from "../primitives/QbzToggle.slint";
 import { QbzSelect } from "../primitives/QbzSelect.slint";
 import { QbzIcon } from "../primitives/QbzIcon.slint";
+import { WarningBanner } from "../primitives/WarningBanner.slint";
 import { SettingRow } from "SettingRow.slint";
 
 component GroupHeader inherits Text {
@@ -95,6 +96,13 @@ export component AudioSettings inherits VerticalLayout {
             }
         }
     }
+    // JACK routes through the JACK graph (resamples to the graph rate), so it
+    // can't be bit-perfect — surface that so the user isn't misled.
+    if SettingsState.backend-is-jack: WarningBanner {
+        variant: "warning";
+        title: @tr("JACK is not bit-perfect");
+        body: @tr("The JACK backend routes audio through the JACK graph, which resamples to the graph's sample rate. For bit-perfect playback use ALSA (direct) or PipeWire with passthrough.");
+    }
     SettingRow {
         label: @tr("Output device");
         description: @tr("The DAC or sound device that receives audio.");

crates/qbz-ui/ui/state.slint

@@ -2804,6 +2804,7 @@ export global SettingsState {
     // Audio — Rust-computed conditional flags driving `if`-gated rows.
     in-out property <bool> backend-is-alsa: false;
     in-out property <bool> backend-is-pipewire: false;
+    in-out property <bool> backend-is-jack: false;
     in-out property <bool> alsa-plugin-is-hw: false;
 
     // --- PLAYBACK ----------------------------------------------------------

crates/qbz/src/settings.rs

@@ -147,6 +147,7 @@ pub struct SettingsSnapshot {
     // Audio — conditional flags.
     backend_is_alsa: bool,
     backend_is_pipewire: bool,
+    backend_is_jack: bool,
     alsa_plugin_is_hw: bool,
     // Playback.
     continue_playback: bool,
@@ -481,6 +482,7 @@ fn build_snapshot(
 
     let backend_is_alsa = active_backend == AudioBackendType::Alsa;
     let backend_is_pipewire = active_backend == AudioBackendType::PipeWire;
+    let backend_is_jack = active_backend == AudioBackendType::Jack;
     let alsa_plugin_is_hw = alsa_plugin == AlsaPlugin::Hw;
     let (out_backend_label, out_mode_label, out_backend_active, out_mode_active) =
         output_labels(&audio);
@@ -525,6 +527,7 @@ fn build_snapshot(
         skip_sink_switch: audio.skip_sink_switch,
         backend_is_alsa,
         backend_is_pipewire,
+        backend_is_jack,
         alsa_plugin_is_hw,
         continue_playback,
         show_context_icon: prefs.show_context_icon,
@@ -596,6 +599,7 @@ pub fn apply_snapshot(window: &AppWindow, snap: SettingsSnapshot) {
     // Audio — conditional flags.
     st.set_backend_is_alsa(snap.backend_is_alsa);
     st.set_backend_is_pipewire(snap.backend_is_pipewire);
+    st.set_backend_is_jack(snap.backend_is_jack);
     st.set_alsa_plugin_is_hw(snap.alsa_plugin_is_hw);
     // Playback.
     st.set_continue_playback(snap.continue_playback);
@@ -721,6 +725,7 @@ fn push_conditional_flags(ctx: &SettingsCtx, weak: &slint::Weak<AppWindow>) {
     let plugin = audio.alsa_plugin.unwrap_or(AlsaPlugin::Hw);
     let is_alsa = backend == AudioBackendType::Alsa;
     let is_pipewire = backend == AudioBackendType::PipeWire;
+    let is_jack = backend == AudioBackendType::Jack;
     let plugin_is_hw = plugin == AlsaPlugin::Hw;
     let plugin_index = ALSA_PLUGINS
         .iter()
@@ -730,6 +735,7 @@ fn push_conditional_flags(ctx: &SettingsCtx, weak: &slint::Weak<AppWindow>) {
         let st = w.global::<SettingsState>();
         st.set_backend_is_alsa(is_alsa);
         st.set_backend_is_pipewire(is_pipewire);
+        st.set_backend_is_jack(is_jack);
         st.set_alsa_plugin_is_hw(plugin_is_hw);
         st.set_alsa_plugin_index(plugin_index);
     });