feat(slint): add Play next to track and album context menus

Add a "Play next" ContextMenuItem above "Add to queue" in every menu
that offers queueing: the album page track menu, the artist page track
menu, and the discover album card menu. Play next uses the list-plus
icon and Add to queue switches to list-end, matching Tauri's ListPlus /
ListEnd icon mapping. The queue sidebar track menu is left untouched.

cc

crates/qbz-slint/ui/discover/AlbumCard.slint

@@ -242,6 +242,13 @@ export component AlbumCard inherits Rectangle {
                     }
                     ContextMenuItem {
                         icon: @image-url("../assets/icons/list-plus.svg");
+                        label: @tr("Play next");
+                        clicked => {
+                            root.media-action("album", root.album.id, "play-next");
+                        }
+                    }
+                    ContextMenuItem {
+                        icon: @image-url("../assets/icons/list-end.svg");
                         label: @tr("Add to queue");
                         clicked => {
                             root.media-action("album", root.album.id, "queue");