fix(chat): prevent poll-triggered reload after send by using server timestamp

The 60s checkForUpdates poll was detecting has_updates due to clock skew
between client and server timestamps. Now the send API returns the server
timestamp, and the frontend uses it for markChannelAsRead — ensuring the
poll sees no updates for own sent messages.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/device_manager.py

@@ -1252,7 +1252,7 @@ class DeviceManager:
                     'id': msg_id,
                 }, namespace='/chat')
 
-            return {'success': True, 'message': 'Message sent', 'id': msg_id}
+            return {'success': True, 'message': 'Message sent', 'id': msg_id, 'timestamp': ts}
 
         except Exception as e:
             logger.error(f"Failed to send channel message: {e}")