fix(social): add image dimensions and alt text to Open Graph tags

[mroderick]

Summary

• Add og:image:width (1200) and og:image:height (630) for proper social preview rendering
• Add og:image:alt for accessibility compliance
• Fix potential nil error when workshop has no host sponsor

Analysis

Reviewed Open Graph implementation against ogp.me specification:

Issues Found

1. Missing image dimensions - Per OGP spec, og:image:width and og:image:height help social platforms optimize image display
2. Missing image alt - Recommended for accessibility when using og:image
3. Potential nil error - workshops/_meta_tags.html.haml:5 called workshop.host.name which would raise an error if host is nil for non-virtual workshops

Changes Made

• Added dimensions (1200x630) matching the actual codebar-social.jpg image
• Added alt text: "codebar - making tech more diverse and welcoming"
• Changed to safe navigation: workshop.host&.name || 'codebar'

[KimberleyCook]

🎉