CheckboxController class

Reactive controller for managing checkbox state with convenient methods.

Extends ValueNotifier to provide state management for checkbox widgets with built-in methods for common state transitions. Supports all three checkbox states: checked, unchecked, and indeterminate.

The controller can be used with ControlledCheckbox for reactive state management or manually to coordinate checkbox behavior across widgets.

Example:

final controller = CheckboxController(CheckboxState.unchecked);

// React to changes
controller.addListener(() {
  print('Checkbox is now: ${controller.value}');
});

// Programmatic control
controller.toggle(); // unchecked -> checked
controller.indeterminate(); // -> indeterminate