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/includes/social_links.html

@@ -0,0 +1,7 @@
+{% if SOCIAL %}
+<ul class="social-links">
+  {% for name, url, icon in SOCIAL %}
+  <li><a href="{{ url }}" rel="me" aria-label="{{ name }}" title="{{ name }}">{% include "icons/" ~ icon ~ ".svg" %}</a></li>
+  {% endfor %}
+</ul>
+{% endif %}