refactor(icons): replace local brand SVGs with FA5 classes

Unify social/OAuth/contact icons on the global Font Awesome 5.15.4 pack,
add aksrt_render_social_links for author hero, enqueue FA on admin SPA,
and delete assets/img/icons.

assets/css/frontend/pages/base-1.css

@@ -205,11 +205,10 @@ html.dark .archive-timeline__badge {
   color: var(--color-ink-3);
   transition: border-color 0.2s, color 0.2s;
 }
-.oauth-btn svg {
-  width: 22px;
-  height: 22px;
+.oauth-btn i {
+  font-size: 22px;
+  line-height: 1;
   display: block;
-  fill: currentColor;
 }
 
 .oauth-btn:hover {