feat(slint): bulk "Make available offline" in the Library multi-select bar

Adds the 6th MultiSelectBar action -> offline_cache::cache_tracks(selected),
mirroring Tauri's bulk caching. Clears the selection after queuing. (Also
retires the cache_tracks dead-code warning.)

cc

crates/qbz-slint/src/main.rs

@@ -4892,6 +4892,16 @@ fn main() -> Result<(), Box<dyn std::error::Error>> {
                         let tracks = favorites::selected_tracks(&w);
                         playback::enqueue_tracks(runtime.clone(), handle.clone(), tracks, true);
                     }
+                    "make-offline" => {
+                        let tracks = favorites::selected_tracks(&w);
+                        offline_cache::cache_tracks(
+                            runtime.clone(),
+                            weak.clone(),
+                            handle.clone(),
+                            tracks,
+                        );
+                        favorites::clear_selection(&w);
+                    }
                     "add-to-playlist" => {
                         let ids = favorites::selected_ids(&w);
                         if !ids.is_empty() {

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

@@ -999,6 +999,7 @@ export component FavoritesView inherits Rectangle {
                         { id: "play-next", label: @tr("Play next"), danger: false, needs-selection: true },
                         { id: "queue", label: @tr("Add to queue"), danger: false, needs-selection: true },
                         { id: "add-to-playlist", label: @tr("Add to playlist"), danger: false, needs-selection: true },
+                        { id: "make-offline", label: @tr("Make available offline"), danger: false, needs-selection: true },
                         { id: "remove-selected", label: @tr("Remove from favorites"), danger: true, needs-selection: true },
                         { id: "clear", label: @tr("Clear"), danger: false, needs-selection: true },
                     ];