feat(slint): Settings Audio/Playback 1:1 with Tauri

Rebuild both Settings panels to match the Tauri app — every option
present, on the correct tab, and fully functional (persisted and
applied to the live player).

Audio: streaming quality, limit-quality-to-device, maximum sample rate,
backend, output device, ALSA plugin, hardware volume, exclusive mode,
reserve DAC, DAC passthrough, force bit-perfect, allow quality
fallback, sync-on-startup, lock output device, and a reset button.
Conditional rows (ALSA plugin, hardware volume, max sample rate, force
bit-perfect, lock output device) are gated on Rust-computed flags.

Playback: continue-after-track, show context icon, gapless, restore
session, resume position, stream uncached, initial buffer size (slider),
streaming only, quality-retry behavior. Note gapless and the streaming
options live on the Playback tab though the qbz-audio struct stores them.

- ui_prefs.rs: JSON store for the frontend-only streaming-quality pref
- QbzSlider primitive for the integer buffer-size control
- volume normalization row removed (Tauri hides it)

Caveat: toggling reserve-DAC persists and is read live, but re-arming
the device reservation guard mid-session needs a player-side API that
does not exist yet; it takes effect on next launch.

cc

crates/qbz-slint/ui/app.slint

@@ -36,6 +36,8 @@ export component AppWindow inherits Window {
     callback media-action(string, string, string);
     callback settings-bool(string, bool);
     callback settings-select(string, int);
+    callback settings-slider(string, int);
+    callback settings-reset();
     callback logout();
     callback close-app();
 
@@ -72,6 +74,12 @@ export component AppWindow inherits Window {
         settings-select(k, i) => {
             root.settings-select(k, i);
         }
+        settings-slider(k, v) => {
+            root.settings-slider(k, v);
+        }
+        settings-reset => {
+            root.settings-reset();
+        }
         logout => {
             root.logout();
         }