When joining a room show already sent chat messages

public/app.js

@@ -235,6 +235,10 @@
         // Seamless reconnection — sync to server state
         isReconnecting = false;
         showConnectionStatus("reconnected");
+        if (msg.chatHistory) {
+          chatMessages.innerHTML = "";
+          msg.chatHistory.forEach((m) => addChatMessage(m.username, m.message, m.timestamp));
+        }
         updateUsers(msg.users);
         if (msg.state) {
           applySync(msg.state);