Add header compose shortcut to GitHub New Issue

[Copilot]

This adds a direct “write post” affordance on the homepage header so creating a new post is one click from the main nav. The new control sits beside the existing GitHub link and routes to the repository’s New Issue page.

• Navigation update

  • Added a dedicated icon link in the header nav (blog/templates/base.html) immediately after the GitHub link.
  • Target URL: {{ repo_url }}/issues/new.
  • Included accessible labeling (aria-label, title) for screen reader and tooltip clarity.

• UI styling

  • Added a focused nav-icon style (.site-nav__icon-link) in blog/static/style.css.
  • Styled as a compact circular control with inherited color and hover border contrast to match existing header behavior.

<a
  href="{{ repo_url }}/issues/new"
  target="_blank"
  rel="noopener noreferrer"
  class="site-nav__icon-link"
  title="Write a new post"
  aria-label="Write a new post"
>
  <svg viewBox="0 0 16 16" aria-hidden="true" focusable="false">…</svg>
</a>