SEO improvements for search engines and AI crawlers

[driveton]

Summary

• OG/Twitter meta tags on leaf pages — sections, pages, and picture leaves now produce rich social previews when shared. Title uses the existing page_title helper, description is pulled from the leaf content (section body, page text, or picture caption); image falls back to book cover or theme default.
• Canonical URLs — added to both book show and leaf show pages so search engines know the authoritative URL.
• /llms.txt — new dynamic endpoint (unauthenticated) listing all published books in the https://llmstxt.org Markdown format, including title, subtitle, and author. Helps AI crawlers understand site structure.