multiple updates

- colour theme implemented. ThemeService based global switching for future light detection triggers
- test flipflop service for various fun
- navigator widget class with state switching

pi/ui/lib/services/test_flipflop_service.dart

@@ -0,0 +1,50 @@
+import 'dart:async';
+
+import 'package:flutter/widgets.dart';
+
+import '../widgets/navigator_widget.dart';
+import 'theme_service.dart';
+
+/// Debug service that flip-flops theme and navigator emotion every 2 seconds.
+///
+/// Usage:
+/// ```dart
+/// TestFlipFlopService.instance.start(navigatorKey: _navigatorKey);
+/// // Later:
+/// TestFlipFlopService.instance.stop();
+/// ```
+class TestFlipFlopService {
+  TestFlipFlopService._();
+  static final instance = TestFlipFlopService._();
+
+  Timer? _timer;
+  bool _running = false;
+
+  bool get isRunning => _running;
+
+  /// Start the flip-flop cycle.
+  /// Pass the navigator's GlobalKey to trigger emotion changes.
+  void start({required GlobalKey<NavigatorWidgetState> navigatorKey}) {
+    stop();
+    _running = true;
+
+    _timer = Timer.periodic(const Duration(seconds: 2), (_) {
+      // Toggle theme
+      ThemeService.instance.toggle();
+
+      // Surprise the navigator
+      if (navigatorKey.currentState?.emotion == 'surprise') {
+        navigatorKey.currentState?.reset();
+      } else {
+        navigatorKey.currentState?.setEmotion('surprise');
+      }
+    });
+  }
+
+  /// Stop the flip-flop cycle.
+  void stop() {
+    _timer?.cancel();
+    _timer = null;
+    _running = false;
+  }
+}

pi/ui/lib/services/theme_service.dart

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+/// Theme switching service - singleton pattern matching OverheatMonitor
+///
+/// Manages dark/bright mode state and notifies listeners on change.
+/// Default is dark mode. Call setDarkMode() from sensor readings.
+class ThemeService {
+  ThemeService._();
+  static final instance = ThemeService._();
+
+  bool _isDarkMode = true;
+  final List<void Function()> _listeners = [];
+
+  /// Current theme mode
+  bool get isDarkMode => _isDarkMode;
+
+  /// Set theme mode. Notifies listeners if changed.
+  void setDarkMode(bool dark) {
+    if (_isDarkMode == dark) return;
+    _isDarkMode = dark;
+    _notifyListeners();
+  }
+
+  /// Toggle between dark and bright
+  void toggle() => setDarkMode(!_isDarkMode);
+
+  /// Add a listener for theme changes
+  void addListener(void Function() listener) {
+    _listeners.add(listener);
+  }
+
+  /// Remove a listener
+  void removeListener(void Function() listener) {
+    _listeners.remove(listener);
+  }
+
+  void _notifyListeners() {
+    for (final listener in _listeners) {
+      listener();
+    }
+  }
+}