feat(slint): add Settings page with Audio and Playback panels

Introduce the Settings page, reachable from the Discover gear and the
header menu. Two sub-sections backed by the existing UI-agnostic stores.

- Audio panel: backend, output device and sample-rate dropdowns;
  toggles for exclusive mode, DAC passthrough, PipeWire force
  bit-perfect, gapless, normalization and quality fallback
- Playback panel: autoplay mode, show context icon, remember session
  and resume position
- settings.rs controller owns AudioSettingsStore + PlaybackPreferencesStore
  at the shared global path; persists changes and applies audio ones to
  the live Player (reload_settings, plus reinit_device for routing-
  critical changes); a backend switch re-enumerates output devices
- QbzToggle (switch) and QbzSelect (PopupWindow dropdown) primitives,
  SettingRow layout helper, chevron-down icon
- SettingsState global, ContentView::settings, settings-bool /
  settings-select callbacks wired through the shell

cc

crates/qbz-slint/Cargo.toml

@@ -21,6 +21,7 @@ image = { version = "0.25", default-features = false, features = ["jpeg", "png"]
 urlencoding = "2"
 open = "5"
 qbz-app = { path = "../qbz-app" }
+qbz-audio = { path = "../qbz-audio" }
 qbz-core = { path = "../qbz-core" }
 qbz-models = { path = "../qbz-models" }
 qbz-cache = { path = "../qbz-cache" }