fix(frontend): restore chat DOM structure and guard addMessage against null

frontend/src/main.ts

@@ -119,7 +119,8 @@ function connectWebSocket(room: string) {
 }
 
 function addMessage(type: string, text: string, sender = "", timestamp = new Date()) {
-  const messages = document.getElementById("messages")!;
+  const messages = document.getElementById("messages");
+  if (!messages) return;
   const div = document.createElement("div");
   div.className = "message-row";