Add MermaidJS diagram rendering for markdown content (READMEs, issues, PRs) using goldmark-mermaid with client-side rendering. Mermaid fenced code blocks are transformed to <pre class="mermaid"> elements, which MermaidJS picks up and renders in the browser. Changes: - Add go.abhg.dev/goldmark/mermaid dependency with client-side render mode - Add mermaid extender before highlighting in goldmark pipeline - Allow "mermaid" class on <pre> elements through bluemonday sanitizer - Conditionally load MermaidJS from CDN only on pages with diagrams - Add basic mermaid container styling with dark mode support - Add tests verifying mermaid blocks produce correct HTML output Closes https://tangled.org/tangled.org/core/issues/424 Signed-off-by: Nate Spilman <nate.spilman@gmail.com> Signed-off-by: Seongmin Lee <git@boltless.me>