Add player labels to normal form game visual

### This feature request is for...

PyGambit

### Requested enahancement or feature

Current output shows a title and a pandas df style table for the payoffs, but it would be useful to add the player labels as row and column headers for clarity:

<img width="729" height="842" alt="Image" src="https://github.com/user-attachments/assets/a7ceb18e-42e8-4a2b-8627-d0e7a05a9cac" />