Removes the light/dark toggle infrastructure in favor of a single dark
theme. Deletes the theme context, boot script, light-mode CSS overrides,
logotype-light asset, and the pierre-light diff theme. Collapses
graph-theme into a single constant. Adds scheme-only-dark on <html> so
native controls and the server-injected Graphviz @media query render
dark.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>