refactor(web): the Mermaid code block lives in its own component file

MarkdownMermaidCodeBlock, with its diagram/source toggle and find reveal,
moves to chat/MarkdownMermaidCodeBlock.tsx next to MermaidDiagram.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
0a9ade9156Julius Marminge committed on 10/21/2026, 8:14:28 AMparenteadb13d

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