Fix SEO gaps: JSON-LD, canonical, and OG templates

BaseLayout.astro has basic og tags but we're missing structured data and a few things that hurt SEO.

Todo:
- add JSON-LD for Article (headline, datePublished, author, image), BreadcrumbList on blog posts, and Person on the homepage/about page
- make sure canonical URL in BaseLayout.astro:15 handles trailing slashes consistently
- check sitemap.xml.ts includes blog tags, archive pages, and projects — right now it only has the main collections
- validate robots.txt.ts with sitemap reference and noindex for preview/draft routes
- review src/pages/open-graph/[...route].ts templates — add per-tag background variant and ensure fonts render in production on Vercel

Test with Google Rich Results and share debugger before merging. No content changes needed, just markup.