UI: Heartbeat Health Dashboard

[rodneyosodo]

Summary

The backend will soon publish rich heartbeat metrics (uptime, memory, device count, MQTT connection state) as part of #25. The UI should surface these in a dedicated health/dashboard section.

What to build

• Add a Health page at /ui/health (or extend the Home page overview section)
• Poll /health or parse the heartbeat SenML data to display:
  • Uptime (formatted as Xd Xh Xm)
  • Memory used / free (bar or gauge)
  • Connected devices count
  • MQTT connection state (online/offline indicator)
  • Agent version string
  • Firmware version string
• Auto-refresh every heartbeat interval
• Add a nav entry in the sidebar/topbar

Depends on

• Rich Heartbeat Payload #25 (Rich Heartbeat Payload) — the backend must expose these metrics first

Reference

Aeolus heartbeat payload format (docs/heartbeat.md):

[
  {"n":"heartbeat","vb":true},
  {"n":"fw_version","vs":"4.2.0"},
  {"n":"uptime","u":"s","v":3600},
  {"n":"heap_free","u":"By","v":12384},
  {"n":"devices","u":"count","v":3},
  {"n":"connected","vb":true}
]