overheat monitor with auto poweroff

- needs passwordless sudo on target machine or deploy to run as root

pi/ui/config.json

@@ -0,0 +1,7 @@
+{
+  "overheat": {
+    "threshold_celsius": 75.0,
+    "trigger_duration_sec": 10,
+    "shutdown_delay_sec": 10
+  }
+}