fix(web): render Mermaid diagrams in Markdown - #6191

Note

Add Mermaid diagram rendering to ChatMarkdown

• ChatMarkdown renders completed mermaid fenced blocks through the new MermaidDiagram component instead of source code. Streaming blocks and failed renders fall back to Shiki.
• MermaidDiagram dynamically loads Mermaid, initializes with strict security and the active theme, and injects the SVG into a scrollable container.
• Clipboard logic in markdown-clipboard.ts copies the original Mermaid source via data-markdown-copy when copying a rendered diagram.
• Behavioral Change: Selections inside a data-markdown-copy ancestor copy explicit Markdown text instead of the rendered fragment. Streaming mermaid blocks render via Shiki.

^{Macroscope summarized cf50ae5.}

Note

Medium Risk
Introduces client-side diagram rendering with injected SVG HTML in chat markdown; mitigated by Mermaid strict security, render fallbacks, and bounded copy serialization, but still expands untrusted-content parsing surface and bundle weight.

Overview
Chat markdown now renders completed mermaid fenced blocks as diagrams instead of Shiki code, while streaming messages still show the normal code path until the fence is finished.

A new MermaidDiagram path lazy-loads mermaid, runs renders on a global serial queue (because Mermaid config is shared), uses strict security settings and the active light/dark theme, and falls back to the existing highlighted code block on failure or while loading. Diagram containers get scroll-friendly layout CSS and the code-block wrap control is hidden when diagram output is shown.

Copy/paste is updated so rendered diagrams expose data-markdown-copy with a safely length-padded fence via serializeMarkdownCodeFence, and clipboard handling prefers that ancestor markdown when a selection only captures inner SVG text.

^{Reviewed by Cursor Bugbot for commit cf50ae5. Bugbot is set up for automated code reviews on this repo. Configure here.}