Closes XSS hole in HtmlBlock by sanitizing user/AI-supplied markup
through DOMPurify before passing to dangerouslySetInnerHTML. Adds
Vitest + jsdom for unit testing with 5 passing tests covering script
stripping, on-event handler removal, javascript: URL blocking, iframe
allowlist, and form/input stripping.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
