Improve ping/pong and offline state

🎯 Major Features Added

1. Broker-Initiated Auto-Ping System

• Broker now actively monitors client health by periodic pinging
• Configurable ping interval (default: 5 seconds)
• Configurable missed ping threshold (default: 2)
• Automatic client disconnect on timeout
• Separate runtime ping state (not stored in flash)

2. Client Pong Callback

• Clients can register onPong() callback to log broker response
• Automatic response to broker pings