Hamburger icon should change to an X when open

In narrow views the left-nav becomes a hamburger:

> <img width="444" height="393" alt="Image" src="https://github.com/user-attachments/assets/299c7a8b-ef63-4385-be1a-56aab05c5685" />

The hamburger should change appearance (show an X) when open, but it does not:

> <img width="444" height="677" alt="Image" src="https://github.com/user-attachments/assets/0b0c7708-ce56-49d1-bcc1-7ff2c6b7e2d3" />