Add social link icons to sidebar and About-me page

Small monochrome icon row (GitHub, LinkedIn, ORCID, Mastodon, Bluesky,
Matrix) rendered from a SOCIAL config tuple via a shared Jinja
partial (templates/includes/social_links.html), included unconditionally
in the sidebar below the tag cloud and again on the About-me page only.
Icons are inline SVGs (templates/icons/*.svg) using fill=currentColor,
so they track the existing accent-color/dark-mode toggle for free via
--main-link-color, no JS needed.

templates/base.html

@@ -113,6 +113,7 @@
       </ul>
       {% endif %}
       {% endif %}
+      {% include "includes/social_links.html" %}
     </nav><!-- /#menu -->
     <main id="content">
     {% block content %}