Implement periodic heartbeat transmission

[xishell]

Description

Transmit a heartbeat message every 5 minutes regardless of state change. The heartbeat should include uptime, battery level, and current occupancy state so the backend can detect offline nodes.

Acceptance Criteria

• Heartbeat is transmitted every 5 minutes even if no state change occurs
• Heartbeat payload includes uptime_s, battery_pct, and current occupied state
• Backend can distinguish heartbeat messages from state-change messages