Add "Fork me on GitHub" corner ribbon

[Copilot]

Adds the classic GitHub Octocat corner ribbon to the site so visitors can easily fork the repository.

Changes

• src/_includes/header.njk — prepends the official SVG ribbon from github.blog/news-insights/github-ribbons:
  • position: fixed; top: 0; right: 0; z-index: 9999 — stays pinned above all elements including the sticky header
  • Waving-arm hover animation via @keyframes octocat-wave; on mobile (≤500px) the animation plays automatically instead
  • target="_blank" rel="noopener noreferrer" + aria-label for accessibility

<a href="https://github.com/NextCommunity/NextCommunity.github.io"
   class="github-corner" aria-label="Fork me on GitHub"
   target="_blank" rel="noopener noreferrer">
  <svg width="80" height="80" viewBox="0 0 250 250"
       style="fill:#151513; color:#fff; position:fixed; top:0; border:0; right:0; z-index:9999;"
       aria-hidden="true">
    …
  </svg>
</a>

Screenshot