The source field was set to "esp32" on the first UDP frame but never
reverted when frames stopped arriving. This caused the UI to show
"Real hardware connected" indefinitely after powering off all nodes.
Changes:
- Add last_esp32_frame timestamp to AppStateInner
- Add effective_source() method with 5-second timeout
- Source becomes "esp32:offline" when no frames received within 5s
- Health endpoint shows "degraded" instead of "healthy" when offline
- All 6 status/health/info API endpoints use effective_source()
Fixes#297
Co-authored-by: Reuven <cohen@ruv-mac-mini.local>
7a13877fa3