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/lib/dataloader.ts

@@ -79,14 +79,23 @@ async function resolveShards(
   const shardResults = await Promise.allSettled(
     index.shards.map(async shard => {
       const url = shard.url.startsWith('http') ? shard.url : `${base}${shard.url}`;
+      // Base URL of this shard's directory (e.g. "http://…/data/dave/_merged/")
+      const shardBase = url.substring(0, url.lastIndexOf('/') + 1);
       const sub = await fetchJSON<BASIndex>(url);
       // Recursively resolve nested shards (e.g. user shard that itself paginates)
       const activities = await resolveShards(sub, url);
-      // Tag each activity with the handle declared in the shard entry
-      if (shard.handle) {
-        return activities.map(a => ({ ...a, handle: shard.handle }));
-      }
-      return activities;
+      // Rewrite relative detail_url / track_url to be absolute so they can be
+      // fetched correctly regardless of where the root index lives.
+      return activities.map(a => ({
+        ...a,
+        ...(shard.handle ? { handle: shard.handle } : {}),
+        detail_url: a.detail_url && !a.detail_url.startsWith('http')
+          ? `${shardBase}${a.detail_url}`
+          : a.detail_url,
+        track_url: a.track_url && !a.track_url.startsWith('http')
+          ? `${shardBase}${a.track_url}`
+          : a.track_url,
+      }));
     }),
   );
 
@@ -152,7 +161,10 @@ export async function loadActivity(
   if (cached) return cached;
 
   try {
-    return await fetchJSON<ActivityDetail>(`${baseUrl}data/${detailUrl}`);
+    const url = detailUrl.startsWith('http')
+      ? detailUrl
+      : `${baseUrl}data/${detailUrl}`;
+    return await fetchJSON<ActivityDetail>(url);
   } catch {
     return null;
   }