feat(slint): add queue sidebar controller

QueueController owns the sidebar-local view state — active tab, search
query, paginator page — and the cached favorite-track IDs. It pulls a
full queue snapshot via QbzCore::get_queue_state_full, applies the search
filter, slices out the current 40-track page, and pushes the result onto
QueueState. Every queue mutation refreshes it.

The four footer actions: clear-queue calls core clear_queue (wiping the
now-playing slot only when nothing is audible, matching Tauri);
save-as-playlist resolves the queue track set (name-input modal pending);
infinite-play toggles the persisted AutoplayMode; search filters the
upcoming list client-side. play paths route through after_track_change,
extracted from the per-call duplication in playback.rs.

cc

crates/qbz-slint/src/settings.rs

@@ -105,6 +105,16 @@ impl SettingsCtx {
             maps: Mutex::new(SettingsMaps::default()),
         })
     }
+
+    /// A handle to the playback-preferences store sharing the same
+    /// underlying SQLite connection. The Queue controller uses it to read
+    /// and toggle the autoplay (infinite-play) mode so the sidebar's
+    /// infinite-play button stays in step with the Playback settings.
+    pub fn playback_prefs(&self) -> PlaybackPreferencesState {
+        PlaybackPreferencesState {
+            store: std::sync::Arc::clone(&self.playback.store),
+        }
+    }
 }
 
 /// Plain, `Send` settings data built off the UI thread.