feat(headlines-feed): implement filter event classes

- Define base HeadlinesFilterEvent class
- Add FilterDataLoaded event for initial filter data load
- Implement FilterTopicToggled, FilterSourceToggled, and FilterCountryToggled events for checkbox toggles
- Add FollowedItemsFilterToggled event for followed items toggle
- Implement FilterSelectionsCleared event to clear all filters

Author: fulleni

lib/headlines-feed/bloc/headlines_filter_event.dart

@@ -0,0 +1,120 @@
+part of 'headlines_filter_bloc.dart';
+
+/// {@template headlines_filter_event}
+/// Base class for all events in the [HeadlinesFilterBloc].
+/// {@endtemplate}
+sealed class HeadlinesFilterEvent extends Equatable {
+  /// {@macro headlines_filter_event}
+  const HeadlinesFilterEvent();
+
+  @override
+  List<Object> get props => [];
+}
+
+/// {@template filter_data_loaded}
+/// Event triggered to load all initial filter data (topics, sources, countries).
+///
+/// This event is dispatched when the filter page is initialized.
+/// {@endtemplate}
+final class FilterDataLoaded extends HeadlinesFilterEvent {
+  /// {@macro filter_data_loaded}
+  const FilterDataLoaded({
+    this.initialSelectedTopics = const [],
+    this.initialSelectedSources = const [],
+    this.initialSelectedCountries = const [],
+    this.isUsingFollowedItems = false,
+  });
+
+  /// The topics that were initially selected on the previous page.
+  final List<Topic> initialSelectedTopics;
+
+  /// The sources that were initially selected on the previous page.
+  final List<Source> initialSelectedSources;
+
+  /// The countries that were initially selected on the previous page.
+  final List<Country> initialSelectedCountries;
+
+  /// Whether the filter is initially set to use followed items.
+  final bool isUsingFollowedItems;
+
+  @override
+  List<Object> get props => [
+    initialSelectedTopics,
+    initialSelectedSources,
+    initialSelectedCountries,
+    isUsingFollowedItems,
+  ];
+}
+
+/// {@template filter_topic_toggled}
+/// Event triggered when a topic checkbox is toggled.
+/// {@endtemplate}
+final class FilterTopicToggled extends HeadlinesFilterEvent {
+  /// {@macro filter_topic_toggled}
+  const FilterTopicToggled({required this.topic, required this.isSelected});
+
+  /// The [Topic] that was toggled.
+  final Topic topic;
+
+  /// The new selection state of the topic.
+  final bool isSelected;
+
+  @override
+  List<Object> get props => [topic, isSelected];
+}
+
+/// {@template filter_source_toggled}
+/// Event triggered when a source checkbox is toggled.
+/// {@endtemplate}
+final class FilterSourceToggled extends HeadlinesFilterEvent {
+  /// {@macro filter_source_toggled}
+  const FilterSourceToggled({required this.source, required this.isSelected});
+
+  /// The [Source] that was toggled.
+  final Source source;
+
+  /// The new selection state of the source.
+  final bool isSelected;
+
+  @override
+  List<Object> get props => [source, isSelected];
+}
+
+/// {@template filter_country_toggled}
+/// Event triggered when a country checkbox is toggled.
+/// {@endtemplate}
+final class FilterCountryToggled extends HeadlinesFilterEvent {
+  /// {@macro filter_country_toggled}
+  const FilterCountryToggled({required this.country, required this.isSelected});
+
+  /// The [Country] that was toggled.
+  final Country country;
+
+  /// The new selection state of the country.
+  final bool isSelected;
+
+  @override
+  List<Object> get props => [country, isSelected];
+}
+
+/// {@template followed_items_filter_toggled}
+/// Event triggered when the "Apply my followed items" button is toggled.
+/// {@endtemplate}
+final class FollowedItemsFilterToggled extends HeadlinesFilterEvent {
+  /// {@macro followed_items_filter_toggled}
+  const FollowedItemsFilterToggled({required this.isUsingFollowedItems});
+
+  /// The new state of the "Apply my followed items" toggle.
+  final bool isUsingFollowedItems;
+
+  @override
+  List<Object> get props => [isUsingFollowedItems];
+}
+
+/// {@template filter_selections_cleared}
+/// Event triggered to clear all active filter selections.
+/// {@endtemplate}
+final class FilterSelectionsCleared extends HeadlinesFilterEvent {
+  /// {@macro filter_selections_cleared}
+  const FilterSelectionsCleared();
+}