Adds help.html to the repo root, resolving the 404 at /help for users
accessing the Nexus web app. The page documents navigation controls,
keyboard bindings, Timmy chat commands, portal status indicators, and
HUD panel descriptions, using the existing dark-space/holographic design
system (Orbitron + JetBrains Mono, teal/purple palette).
Also adds tests/test_help_page.py to verify the page exists and covers
required sections.
Fixes#833
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>