feat(slint): favorites tracks search + results count

Add an expandable search over the loaded favorite tracks (title /
artist / album) plus a results-count line. tracks-visible (what the list
renders) derives from the full tracks set: an empty query shares the
model so artwork stays live, a query forks a filtered clone carrying the
already-decoded artwork (the LabelState albums/visible pattern). Adds a
no-match empty state.

cc

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

@@ -20,6 +20,7 @@ import {
 } from "../state.slint";
 import { AlbumCard } from "../discover/AlbumCard.slint";
 import { TrackRow } from "../primitives/TrackRow.slint";
+import { ExpandableSearch } from "../primitives/ExpandableSearch.slint";
 import { QbzIcon } from "../primitives/QbzIcon.slint";
 import { LoadingSpinner } from "../primitives/LoadingSpinner.slint";
 import { ListScrollbar } from "../primitives/ListScrollbar.slint";
@@ -333,12 +334,48 @@ export component FavoritesView inherits Rectangle {
             if !FavoritesState.loading
                 && FavoritesState.active-tab == "tracks": VerticalLayout {
                 spacing: 0px;
+                // Toolbar — results count (left) + expandable search (right).
+                if FavoritesState.tracks.length > 0: HorizontalLayout {
+                    spacing: 8px;
+                    VerticalLayout {
+                        alignment: center;
+                        horizontal-stretch: 1;
+                        Text {
+                            text: FavoritesState.tracks-visible.length
+                                + (FavoritesState.tracks-search != ""
+                                    ? " / " + FavoritesState.tracks.length
+                                    : "")
+                                + " " + @tr("tracks");
+                            color: Theme.text-muted;
+                            font-size: Typography.legal;
+                        }
+                    }
+                    VerticalLayout {
+                        alignment: center;
+                        ExpandableSearch {
+                            placeholder: @tr("Search");
+                            query <=> FavoritesState.tracks-search;
+                            edited(q) => {
+                                FavoritesActions.search-tracks(q);
+                            }
+                        }
+                    }
+                }
+                if FavoritesState.tracks.length > 0: Rectangle { height: 12px; }
+
                 if FavoritesState.tracks.length == 0: Text {
                     text: @tr("No favorite tracks yet.");
                     color: Theme.text-muted;
                     font-size: Typography.body;
                 }
-                for track[i] in FavoritesState.tracks: TrackRow {
+                // Search filtered everything out.
+                if FavoritesState.tracks.length > 0
+                    && FavoritesState.tracks-visible.length == 0: Text {
+                    text: @tr("No tracks match your search.");
+                    color: Theme.text-muted;
+                    font-size: Typography.body;
+                }
+                for track[i] in FavoritesState.tracks-visible: TrackRow {
                     track: track;
                     index: i;
                     show-artwork: true;

crates/qbz-slint/ui/state.slint

@@ -1447,8 +1447,13 @@ export global FavoritesState {
     // "tracks" | "albums" | "artists" | "playlists" | "labels"
     in-out property <string> active-tab: "tracks";
     // Tracks reuse TrackItem so the shared SearchTrackRow can
-    // render them (artwork-on, per-row cover thumbnail).
+    // render them (artwork-on, per-row cover thumbnail). `tracks` is the
+    // full fetched set (the artwork pipeline targets it); `tracks-visible`
+    // is what the list renders after the search filter (mirrors
+    // LabelState.albums vs .visible).
     in property <[TrackItem]> tracks: [];
+    in property <[TrackItem]> tracks-visible: [];
+    in-out property <string> tracks-search: "";
     in property <[AlbumCardItem]> albums: [];
     in property <[FavoriteArtistItem]> artists: [];
     in property <[FavoritePlaylistItem]> playlists: [];
@@ -1471,6 +1476,7 @@ export global FavoritesActions {
     callback open-playlist(string /* playlist id */);
     callback play-track(string /* track id */);
     callback track-action(string /* track id */, string /* action */);
+    callback search-tracks(string /* query */);
 }
 
 // A Qobuz mix detail page (DailyQ / WeeklyQ / FavQ / TopQ), opened