fix: no attachment broadcast fix

lib/chat-format.ts

@@ -29,10 +29,15 @@ export function buildEntityTree(
   body: string,
   entities: MessageEntity[],
 ): EntityNode[] {
+  // Defensive on both inputs: this renders content from the network, and a
+  // missing body should show nothing rather than take down the screen it is
+  // on. The broadcast modal renders through here too, and it mounts on every
+  // page — so a crash here is not confined to chat.
+  const text = body ?? "";
   const sorted = [...(entities ?? [])].sort(
     (a, b) => a.offset - b.offset || b.length - a.length,
   );
-  return build(body, 0, body.length, sorted);
+  return build(text, 0, text.length, sorted);
 }
 
 function build(