Fix help menu: center it, sans-serif font, correct key casing

Apply pycortex-roidraw fixups.py to viewer.html:
- center #helpmenu (was pinned left:0%, lower half hidden behind the legend)
- give #helpmenu a sans-serif font-family (was falling back to the browser-default serif)
- render shortcut keys in their true case (pycortex force-uppercased them, so 'r' showed as 'R', a different command)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: jackgallant