Drop the duplicate mermaid-block.tsx (own mermaid.initialize + render path,
theme frozen at first load) and wire preview-file.tsx's MarkdownCode through
the existing RichCodeBlock registry from #52935 instead. One mermaid init
path, theme-flip re-init, Zoomable + copy-as-PNG, RichBoundary error
fallback — and the preview pane gets svg fences for free. Shiki block stays
as the fallback for all other languages.
7ff86f4458