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/state.slint

@@ -1411,6 +1411,7 @@ export struct PlaylistPickItem {
 export global PlaylistPickerState {
     in-out property <bool> open: false;
     in property <string> track-id;
+    in property <[string]> track-ids: []; // bulk add (favorites multi-select)
     in property <[PlaylistPickItem]> playlists: [];
     in property <bool> loading: false;
 }