feat(slint): wire play-next media action for tracks and albums

Add play_track_next and enqueue_album_next to playback.rs, calling the
core's add_track_next to insert after the current track. Album play-next
inserts tracks in reverse so they land in order, mirroring Tauri's
v2_add_tracks_to_queue_next. Route ("track"/"album", "play-next")
through the on_media_action handler. Add the lucide list-end icon used
for the existing "Add to queue" item.

cc

crates/qbz-slint/ui/assets/icons/list-end.svg

@@ -0,0 +1,7 @@
+<svg xmlns="http://www.w3.org/2000/svg" width="24" height="24" viewBox="0 0 24 24" fill="none" stroke="#ffffff" stroke-width="2" stroke-linecap="round" stroke-linejoin="round">
+  <path d="M16 12H3" />
+  <path d="M16 6H3" />
+  <path d="M10 18H3" />
+  <path d="M21 18V8a2 2 0 0 0-2-2h-5" />
+  <path d="m16 8-2-2 2-2" />
+</svg>