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.

static/css/site.css

@@ -386,6 +386,37 @@ ul.menuitems {
   content: "Translations: ";
 }
 
+.social-links {
+  display: flex;
+  flex-wrap: wrap;
+  gap: 0.75rem;
+  list-style: none;
+  margin: 3rem 0 0 0;
+}
+
+.social-links li {
+  margin: 0;
+}
+
+.social-links a {
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  color: var(--main-link-color);
+  opacity: 0.85;
+  transition: opacity 0.15s ease;
+}
+
+.social-links a:hover,
+.social-links a:focus-visible {
+  opacity: 1;
+}
+
+.social-links svg {
+  width: 1.25rem;
+  height: 1.25rem;
+}
+
 
 .menuitems {
   list-style: none;