overheat monitor with auto poweroff

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

pi/ui/lib/services/overheat_monitor.dart

@@ -0,0 +1,77 @@
+import 'dart:async';
+
+import 'config_service.dart';
+import 'pi_io.dart';
+
+/// Monitors CPU temperature and triggers overheat condition
+///
+/// Singleton pattern. Polls temp at 500ms intervals to match dashboard.
+/// When temp exceeds threshold for trigger duration, fires callback.
+class OverheatMonitor {
+  OverheatMonitor._();
+  static final instance = OverheatMonitor._();
+
+  Timer? _timer;
+  int _consecutiveOverheatSamples = 0;
+  bool _triggered = false;
+
+  static const Duration _pollInterval = Duration(milliseconds: 500);
+
+  /// Current temperature (from PiIO cache)
+  double? get currentTemp => PiIO.instance.getTemperature();
+
+  /// Whether overheat condition has triggered
+  bool get isTriggered => _triggered;
+
+  /// How long we've been over threshold
+  Duration get timeOverThreshold =>
+      Duration(milliseconds: _consecutiveOverheatSamples * _pollInterval.inMilliseconds);
+
+  /// Start monitoring with callback when overheat triggers
+  ///
+  /// [onOverheat] fires once when temp exceeds threshold for configured duration.
+  /// Safe to call multiple times - restarts monitoring.
+  void start({required VoidCallback onOverheat}) {
+    stop();
+    _triggered = false;
+    _consecutiveOverheatSamples = 0;
+
+    final threshold = ConfigService.instance.overheatThreshold;
+    final triggerDuration = ConfigService.instance.overheatTriggerDuration;
+
+    _timer = Timer.periodic(_pollInterval, (_) {
+      if (_triggered) return; // Already fired
+
+      final temp = PiIO.instance.getTemperature();
+      if (temp == null) return; // No reading yet
+
+      if (temp > threshold) {
+        _consecutiveOverheatSamples++;
+
+        final overThresholdTime = timeOverThreshold;
+        if (overThresholdTime >= triggerDuration) {
+          _triggered = true;
+          onOverheat();
+        }
+      } else {
+        // Temp dropped below threshold - reset counter
+        _consecutiveOverheatSamples = 0;
+      }
+    });
+  }
+
+  /// Stop monitoring
+  void stop() {
+    _timer?.cancel();
+    _timer = null;
+  }
+
+  /// Reset state (for testing/recovery)
+  void reset() {
+    _triggered = false;
+    _consecutiveOverheatSamples = 0;
+  }
+}
+
+/// Callback signature for void functions
+typedef VoidCallback = void Function();