In case it can affect someone else, I had to reinstall Erlang with wx following the steps from this Gihub issue (Unable to install Erlang/OTP 27.1 and 26.2.5.3 on Apple Silicon macOS and Xcode 16.0 · Issue #319 · asdf-vm/asdf-erlang · GitHub), and now I can:
iex -S mix
> Mix.ensure_application! :observer
> :observer.start
