Dropdown: render submenu icon position conditionally

Currently the submenu may open on either side, but the icon is hardcoded to be on the right side:

<img width="460" alt="Image" src="https://github.com/user-attachments/assets/e90e7575-f788-4736-9845-08da8cb5959b" />

It would be useful to position the icon depending on the side the submenu is opening on.