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/pages/activity/[id].astro

@@ -19,13 +19,26 @@ export async function getStaticPaths() {
     const root: BASIndex = JSON.parse(readFileSync(join(dataDir, 'index.json'), 'utf-8'));
 
     // Collect activities from root (single-user) or walk shards (multi-user)
-    function readActivities(indexPath: string): ActivitySummary[] {
+    function readActivities(indexPath: string, urlPrefix: string = ''): ActivitySummary[] {
       try {
         const idx: BASIndex = JSON.parse(readFileSync(indexPath, 'utf-8'));
-        const own = idx.activities ?? [];
+        const own = (idx.activities ?? []).map(a =>
+          urlPrefix
+            ? {
+                ...a,
+                detail_url: a.detail_url && !a.detail_url.startsWith('http') ? `${urlPrefix}${a.detail_url}` : a.detail_url,
+                track_url:  a.track_url  && !a.track_url.startsWith('http')  ? `${urlPrefix}${a.track_url}`  : a.track_url,
+              }
+            : a
+        );
         const fromShards = (idx.shards ?? []).flatMap(s => {
           const shardPath = join(dataDir, s.url);
-          return readActivities(shardPath);
+          // Prefix for activities read from this shard: path of the shard dir relative to dataDir
+          const shardDir = s.url.substring(0, s.url.lastIndexOf('/') + 1);
+          return readActivities(shardPath, shardDir).map(a => ({
+            ...a,
+            ...(s.handle && !a.handle ? { handle: s.handle } : {}),
+          }));
         });
         return [...own, ...fromShards];
       } catch {