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/theme_service.dart

@@ -0,0 +1,40 @@
+/// 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();
+    }
+  }
+}