Add multi-language syntax highlighting (#16)

Splash (Swift-only, since removed) left code rendering unhighlighted
everywhere. Add Highlightr (highlight.js) behind a small SyntaxHighlighter
service and use it on both surfaces:

- Native file viewer: highlight NSAttributedString by language inferred
  from the filename, picking a light/dark theme from the trait style and
  re-highlighting on style changes. Falls back to plain text for unknown
  languages or very large files.
- Markdown/org code blocks in the WKWebView (READMEs, tickets, comments):
  the web view disables JavaScript, so highlight server-side into inline
  color-styled spans. The color scheme is now part of the render/cache
  key since colors are baked into the HTML.

Covered by unit tests for language mapping and highlighted output.

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