ActivityFeed — replaced the <button> inside <a> (invalid HTML, unreliable) with the stretched-link pattern: the card is now a <div>, the title <a> carries a ::before pseudo-element

that covers the whole card making it clickable, and @handle is a proper <a> with z-index: 10 sitting above the stretched link. Clicking the handle navigates to /u/{handle}/; clicking
  anywhere else navigates to the activity.

  ActivityDetail — @handle link added in the date/time row of the header, linking to /u/{handle}/. Only shown when activity.handle is set (i.e. multi-user mode).

site/src/components/ActivityDetail.svelte

@@ -41,7 +41,9 @@
     localDescription = e.detail.description;
   }
 
-  $: trackUrl = activity.track_url ? `${base}data/${activity.track_url}` : null;
+  $: trackUrl = activity.track_url
+    ? (activity.track_url.startsWith('http') ? activity.track_url : `${base}data/${activity.track_url}`)
+    : null;
   $: color = sportColor(activity.sport);
 
   function lightboxPrev() { if (lightboxIndex !== null) lightboxIndex = (lightboxIndex - 1 + galleryImages.length) % galleryImages.length; }
@@ -169,7 +171,7 @@
         </span>
       {/if}
       <span class="text-xs text-zinc-500">
-        {formatDate(activity.started_at)} · {formatTime(activity.started_at)}
+        {formatDate(activity.started_at)} · {formatTime(activity.started_at)}{#if activity.handle} · <a href="{base}u/{activity.handle}/" class="hover:text-zinc-300 transition-colors">@{activity.handle}</a>{/if}
       </span>
     </div>
     <div class="flex items-center gap-3">