Add 'press h for help' hint to the help menu

Backfill the bottom-center discoverability hint from pycortex-roidraw fixups.py (built into the #main template). The case/centering/font fixes were already applied in earlier commits.

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

Author: jackgallant