fix(queue): wire the Remove all after and Add to playlist row actions

The per-track context menu in the Queue sidebar had two placeholder items
with no click handler: "Remove all after" and "Add to playlist". Wire both
end to end.

- QueueState gains remove-all-after(int) and add-to-playlist(int), resolved
  from the page-local row index like remove-upcoming (search-filter aware).
- Core/player gain remove_upcoming_after(upcoming_index): truncates the
  upcoming list in PLAY order (shuffle-aware, reuses remove_upcoming_track),
  clears the stop-after marker if it lands in the removed range, and emits a
  single QueueUpdated. The pre-existing remove_after (absolute-index, not
  shuffle-aware) was dead code and stays only for its unit coverage.
- Add to playlist reuses the save-as-playlist picker handoff seeded with the
  single row's track id.

cc

crates/qbz-ui/ui/state.slint

@@ -4418,6 +4418,12 @@ export global QueueState {
     callback play-history(int);
     // Remove the upcoming entry at `index` within the current page slice.
     callback remove-upcoming(int);
+    // Remove every upcoming entry after the page-local `index` (that row is
+    // kept). The Rust controller resolves it to a queue-wide upcoming index.
+    callback remove-all-after(int);
+    // Open the Add-to-Playlist picker seeded with the upcoming row at the
+    // page-local `index`.
+    callback add-to-playlist(int);
     // Reorder the upcoming list: move the page-local row `from` to the insertion
     // slot `to` (0 = before the first upcoming row, N = after the last). The Rust
     // controller resolves the page-local indices to queue-wide upcoming indices