Fix show-on-scroll header and social links

[Llewellynvdm]

Summary

• make the shared header visible at the top, hide on downward scroll, and reveal on upward scroll
• keep the header visible while the mobile menu is open or keyboard focus is inside it
• make GitHub and Telegram the only canonical Social Outreach links
• replace the screenshot-derived footer skyline crop with the owner-supplied 1920 × 1280 original
• reduce directory-link sibling spacing from 10px to 5px in both portable and YOOtheme-compatible markup
• update source, built distribution files, examples, documentation, and validation

Validation

• npm run check
• GitHub Actions: Validate theme passing

Historical page-source evidence remains unchanged; the canonical theme follows the repository owner's current instructions.