feat: add Open Graph and Twitter Card meta tags for social sharing
Implement comprehensive OG and Twitter Card support:
- og:title, og:description, og:image, og:type (website/article)
- og:article:published_time and og:article:author for article pages
- twitter:card with summary_large_image, twitter:title, twitter:description, twitter:image
- Per-page description from excerpt field with fallback to site description
- Article-specific tags only rendered for pages with dates
- Default dark thumbnail fallback when article has no image
- Empty twitterHandle param (optional fill-in for users)
Extracted head meta into new partial for maintainability.
