Add a confirmation prompt when emptying the trash

To match up with the other apps, we should show an `Are you sure?` type prompt when emptying the trash, because the action permanently deletes notes.

Start by locating the macOS trash-emptying action and inspect how other apps or existing destructive actions present confirmation prompts. Done means the permanent deletion is preceded by an “Are you sure?” prompt with clear confirmation and cancellation behavior.

Written by the indexing model from the issue text.