Privacy-friendly analytics and an SEO sweep

Switched to Plausible for analytics. No cookies, no tracking scripts phoning home, GDPR-compliant out of the box, and the whole script is about 1KB. Also did a full SEO metadata audit — added OpenGraph and Twitter cards to every service page, the notes section, and the donate page. Blocked the portal from search indexing since that is authenticated content. Updated the sitemap to include pages that were missing. Small things but they add up.