fix: fallback to direct file fetch in ActivityDetailLoader

If the index-based lookup fails (shard fetch silently failed, stale
index state, etc.), try fetching the activity detail file directly from
each user shard's _merged/activities/ directory. This makes private
activities and newly-synced activities accessible even when the index
resolution fails.

Also add console.error logging when shards fail in resolveShards to
help diagnose root causes.

site/src/lib/dataloader.ts

@@ -99,6 +99,13 @@ async function resolveShards(
     }),
   );
 
+  // Log shard fetch failures to help diagnose missing-activity issues
+  shardResults.forEach((r, i) => {
+    if (r.status === 'rejected') {
+      console.error('[bincio] shard fetch failed:', index.shards[i]?.url, r.reason);
+    }
+  });
+
   const own = index.activities ?? [];
   const fromShards = shardResults.flatMap(r => r.status === 'fulfilled' ? r.value : []);
   return [...own, ...fromShards];