feat: added mermaid rendering support in markdown

[tdurieux]

Summary

Rebased version of #489 by @souhailaS to resolve merge conflicts and allow merging.

• Adds mermaid diagram rendering support in markdown (flowcharts, sequence diagrams, pie charts, state diagrams)
• Adds mermaid.min.js and marked-mermaid.js extensions
• Adds mermaid.css for diagram styling
• Updates gulpfile.js to include mermaid assets in the build
• Registers the mermaid extension in renderMD() while preserving the DOMPurify.sanitize() wrapper from main

Conflict resolution

• public/script/utils.js: Combined the PR's marked.use(markedMermaid()) with main's DOMPurify.sanitize() wrapper
• public/css/all.min.css and public/script/bundle.min.js: Rebuilt via gulp to include the new mermaid assets

Closes #489

https://claude.ai/code/session_01VcoDio6aDX7VWni8YizF86