fix(gateway): extract content from payload in received messages

Display payload.content instead of raw JSON when the payload
is an object with a content field.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/gateway/public/index.html

@@ -674,7 +674,8 @@
       });
 
       socket.on('receive', (msg) => {
-        const content = typeof msg.payload === 'string' ? msg.payload : JSON.stringify(msg.payload);
+        const p = msg.payload;
+        const content = typeof p === 'string' ? p : (p && p.content ? p.content : JSON.stringify(p));
         appendMsg('in', content, msg.from);
       });