fix: add syntax highlighting to blog code blocks

[ajbozarth]

Adds rehype-highlight to the blog post renderer and imports the highlight.js GitHub theme (dark by default, light via prefers-color-scheme).

Previously, fenced code blocks in blog posts rendered as plain unstyled text.

[psschwei]

Review Summary

Small, focused PR — the approach is correct. rehype-highlight in the rehype pipeline is the right integration for a statically-exported Next.js site. Three items to address before merging:

1. Add ignoreMissing: true to rehypeHighlight

If a blog author uses a fenced code block with an unrecognized language (e.g. ```mermaid, ```pseudocode), the build will fail. Since blog posts come from external contributors, this needs a safety net:

rehypePlugins={[rehypeSlug, [rehypeHighlight, { ignoreMissing: true }]]}

2. Conditional @import may not survive the Next.js build pipeline

The syntax @import 'github.css' (prefers-color-scheme: light) is valid modern CSS, but the bundler (PostCSS/lightningcss) may flatten it, causing the light theme to unconditionally override the dark theme for all users. Either:

• Verify in the built output that the light-mode styles are wrapped in @media (prefers-color-scheme: light), or
• Switch to importing only github-dark.css and inlining the light-mode color overrides inside the existing @media (prefers-color-scheme: light) block in globals.css (more robust, consistent with how the project already handles theming)

3. Add a defensive CSS reset for .prose pre code.hljs

highlight.js sets padding: 1em, display: block, and overflow-x: auto on pre code.hljs. These apply unchallenged by the existing .prose pre code rule and will cause double padding and layout changes. Add:

.prose pre code.hljs {
  background: none;
  padding: 0;
  display: initial;
  overflow-x: visible;
}

What's working well

• Minimal diff, correct plugin ordering, dependencies already in package.json
• Theme choice (github-dark / github) aligns with the project's dark-first convention and existing --code-bg: #0d1117

[psschwei]

edit: Gemini reviewed the wrong PR ☹️

[ajbozarth]

Thanks for the review! Addressing each item:

1. ignoreMissing: true — Fixed in 5a6bf30.

2. Conditional @import — Tested against the production build (npm run build && npx serve out) and verified the light theme applies correctly under prefers-color-scheme: light. The Next.js 16 / Turbopack pipeline preserves the media condition on the import, so no change needed here.

3. .prose pre code.hljs padding reset — The existing .prose pre code rule in globals.css already sets padding: 0 and background: none with higher specificity than .hljs, so there's no double-padding in practice. No change needed.