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 ----------------------------------------------------------