Fix null-safe username search in block list

src/components/BlockListPanel.tsx

@@ -27,7 +27,7 @@ export default function BlockListPanel({
     if (!q) return blocks;
     return blocks.filter(
       (b) =>
-        b.username.toLowerCase().includes(q) ||
+        (b.username || '').toLowerCase().includes(q) ||
         (b.display_name || '').toLowerCase().includes(q)
     );
   }, [blocks, search]);