refactor(web-ui): split render.ts sanitize, mermaid, and SVG action layers

[lidge-jun]

Summary

public/js/render.ts is still a 995-line mixed-responsibility file that combines markdown rendering, sanitization, Mermaid orchestration, and SVG action UI.

Current Review Status

• Open after the 2026-04-18 pull audit
• public/js/render.ts remains 995L
• No public/js/render/* split modules exist yet

Suggested Scope

• Extract sanitize hooks, Mermaid lifecycle, SVG action helpers, and final markdown entrypoint into separate modules while preserving renderMarkdown() as the public API

Evidence

• public/js/render.ts