artifact embed

every artifact used to require a full netlify deploy just to show a demo on the site. now there's an artifactHtml field on each project — paste raw HTML into the admin form, save, and it renders via iframe srcdoc. no build step, no deploy credits, no new URL to manage. the demo tab detects it automatically and shows "embedded artifact" in the toolbar. existing URL-based embeds still work exactly as before. includes a terminal snake game as the first embedded artifact to prove the concept.