Vendor all site assets for air-gapped build and runtime

[Copilot]

The site still depended on external asset sources and a networked frontend install path, which breaks both build and serving in an air-gapped environment. This change vendors the required theme/runtime assets into the repository and updates Hugo/Docker wiring so generated pages resolve only local resources.

• Build path

  • Removes the container-time npm ci step and switches the Hugo build to consume repository-vendored dependencies directly.
  • Keeps the build fail-fast if node_modules/ is missing, making the offline assumption explicit.

• Hugo asset mounts

  • Extends config/_default/module.toml to mount vendored third-party assets into Hugo:
    • mermaid under assets/vendor/mermaid
    • katex under static/vendor/katex
    • mathjax under static/vendor/mathjax

• Template overrides for local-only resources

  • Replaces CDN-backed KaTeX and MathJax partials with local asset references.
  • Overrides Mermaid initialization so the generated script imports the vendored module instead of cdn.jsdelivr.net.

• Repository packaging

  • Stops ignoring node_modules/ so the theme packages, fonts, icons, and JS dependencies are present in-repo for offline builds.
  • Adds the required runtime packages (katex, mathjax, mermaid) to the project dependency set so their distributable assets are versioned alongside the site.

• Docs

  • Updates the README and introduction docs to reflect that frontend/theme assets are vendored for air-gapped use and no network fetch is required during build.

<link rel="stylesheet" href="{{ "vendor/katex/katex.min.css" | absURL }}">
<script defer src="{{ "vendor/mathjax/tex-mml-chtml.js" | absURL }}"></script>