feat: add Mermaid diagram rendering to all markdown views

[PengyiZhang]

Summary

• Add Mermaid diagram rendering to wiki reader (wiki-reader.tsx), file preview (file-preview.tsx), and research panel (research-panel.tsx)
• Reuses the existing MermaidDiagram component (IntersectionObserver lazy render + click-to-zoom overlay)
• Consistent behavior with the chat mermaid support added in feat: add Mermaid diagram rendering in chat #101

Test plan

• Open a wiki page containing a ```mermaid fenced code block — verify diagram renders as SVG
• Open a markdown file in the preview pane with mermaid code — verify diagram renders
• Run a Deep Research task that produces mermaid output — verify diagram renders in synthesis
• Click a rendered diagram in any view — verify zoom overlay opens with +/- controls
• Verify non-mermaid code blocks still render normally in all views

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