feat: add Mermaid diagram rendering in chat

[PengyiZhang]

Summary

• Add Mermaid diagram rendering support in chat messages using react-markdown code block detection
• Render diagrams lazily via IntersectionObserver to avoid layout shift on scroll
• Support click-to-zoom overlay with +/- scaling controls for large diagrams

Test plan

• Send a chat message containing a ```mermaid fenced code block (e.g. graph TD, sequenceDiagram) and verify it renders as an SVG diagram
• Scroll a page with multiple mermaid diagrams — verify no layout jumping
• Click a rendered diagram — verify fullscreen overlay opens with zoom controls
• Test zoom in/out buttons and Esc to close
• Verify non-mermaid code blocks still render normally

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