feat(slint): favorites bulk Play next / Add to queue / Add to playlist

Extend the favorites tracks bulk bar beyond Remove: Play next and Add to
queue build the queue from the already-loaded Track objects (new
playback::enqueue_tracks, no re-fetch; play-next inserts in order), and
Add to playlist opens the picker for the whole selection. The picker
gains track-ids for bulk add (open_multi); on_pick adds every id via the
existing add_tracks_to_playlist. Make-offline stays out (no Slint
offline-cache path yet).

cc

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

@@ -546,6 +546,9 @@ export component FavoritesView inherits Rectangle {
                     selected-count: FavoritesState.tracks-selected-count;
                     actions: [
                         { id: "select-all", label: @tr("Select all"), danger: false, needs-selection: false },
+                        { 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: "remove-selected", label: @tr("Remove from favorites"), danger: true, needs-selection: true },
                         { id: "clear", label: @tr("Clear"), danger: false, needs-selection: true },
                     ];