I6: Landing SEO + schema.org + OG tags

[Dgetsylver]

Scope
landing/index.html is missing Open Graph tags, Twitter card tags, and JSON-LD for schema.org. Add them.

Acceptance

• Preview checks pass on the Meta and Twitter card validators.
• Lighthouse SEO score >= 95.

Files
landing/index.html

[Sikkra]

Claiming this one. I will add Open Graph, Twitter Card, canonical/robots metadata, and schema.org JSON-LD to landing/index.html.