feat(slint): search box for the favorites Labels tab

Add an ExpandableSearch to the Labels toolbar (mirrors tracks/albums),
wired to labels-search + the new derive_labels controller fn, which
forks a name-filtered clone of the loaded labels and shares the full
model when the query is empty (artwork stays live). apply_favorites now
derives the visible set on load.

cc

crates/qbz-slint/ui/favorites/FavoritesView.slint

@@ -614,6 +614,21 @@ export component FavoritesView inherits Rectangle {
                     ViewToggle { }
                 }
             }
+            if FavoritesState.active-tab == "labels"
+                && FavoritesState.labels.length > 0: HorizontalLayout {
+                spacing: 8px;
+                VerticalLayout {
+                    alignment: center;
+                    ExpandableSearch {
+                        placeholder: @tr("Search");
+                        sm: true;
+                        query <=> FavoritesState.labels-search;
+                        edited(q) => {
+                            FavoritesActions.search-labels(q);
+                        }
+                    }
+                }
+            }
         }
 
         Rectangle { height: 14px; }

crates/qbz-slint/ui/state.slint

@@ -1489,6 +1489,7 @@ export global FavoritesState {
     in property <[FavoritePlaylistItem]> playlists: [];
     in property <[FavoriteLabelItem]> labels: [];
     in property <[FavoriteLabelItem]> labels-visible: []; // after search filter (rendered)
+    in-out property <string> labels-search: "";
     in property <int> tracks-total: 0;
     in property <int> albums-total: 0;
     in property <int> artists-total: 0;
@@ -1516,6 +1517,7 @@ export global FavoritesActions {
     callback bulk-action(string /* select-all | remove-selected | clear */);
     callback album-action(string /* album id */, string /* action */);
     callback albums-search(string /* query */);
+    callback search-labels(string /* query */);
     callback albums-set-view(string /* grid | list */);
     callback albums-set-sort(string /* sort key */);
     callback albums-set-group(string /* off | alpha | artist */);