feat(headlines-feed): implement followed sources filter functionality

- Add AppBloc and UserContentPreferences repository dependencies to SourcesFilterBloc
- Implement _onSourcesFilterApplyFollowedRequested event handler
- Add loading, success, and failure states for followed sources
- Handle unauthorized and empty followed sources scenarios
- Update SourcesFilterState to include followed sources and related status

Author: fulleni

lib/headlines-feed/bloc/sources_filter_bloc.dart

@@ -4,6 +4,7 @@ import 'package:bloc/bloc.dart';
 import 'package:core/core.dart';
 import 'package:data_repository/data_repository.dart';
 import 'package:equatable/equatable.dart';
+import 'package:flutter_news_app_mobile_client_full_source_code/app/bloc/app_bloc.dart'; // Import AppBloc
 
 part 'sources_filter_event.dart';
 part 'sources_filter_state.dart';
@@ -12,20 +13,29 @@ class SourcesFilterBloc extends Bloc<SourcesFilterEvent, SourcesFilterState> {
   SourcesFilterBloc({
     required DataRepository<Source> sourcesRepository,
     required DataRepository<Country> countriesRepository,
-  }) : _sourcesRepository = sourcesRepository,
-       _countriesRepository = countriesRepository,
-       super(const SourcesFilterState()) {
+    required DataRepository<UserContentPreferences>
+        userContentPreferencesRepository, 
+    required AppBloc appBloc,
+  })  : _sourcesRepository = sourcesRepository,
+        _countriesRepository = countriesRepository,
+        _userContentPreferencesRepository = userContentPreferencesRepository,
+        _appBloc = appBloc,
+        super(const SourcesFilterState()) {
     on<LoadSourceFilterData>(_onLoadSourceFilterData);
     on<CountryCapsuleToggled>(_onCountryCapsuleToggled);
     on<AllSourceTypesCapsuleToggled>(_onAllSourceTypesCapsuleToggled);
     on<SourceTypeCapsuleToggled>(_onSourceTypeCapsuleToggled);
     on<SourceCheckboxToggled>(_onSourceCheckboxToggled);
     on<ClearSourceFiltersRequested>(_onClearSourceFiltersRequested);
-    // Removed _FetchFilteredSourcesRequested event listener
+    on<SourcesFilterApplyFollowedRequested>(
+      _onSourcesFilterApplyFollowedRequested,
+    );
   }
 
   final DataRepository<Source> _sourcesRepository;
   final DataRepository<Country> _countriesRepository;
+  final DataRepository<UserContentPreferences> _userContentPreferencesRepository;
+  final AppBloc _appBloc;
 
   Future<void> _onLoadSourceFilterData(
     LoadSourceFilterData event,
@@ -182,6 +192,65 @@ class SourcesFilterBloc extends Bloc<SourcesFilterEvent, SourcesFilterState> {
     );
   }
 
+  /// Handles the request to apply the user's followed sources as filters.
+  Future<void> _onSourcesFilterApplyFollowedRequested(
+    SourcesFilterApplyFollowedRequested event,
+    Emitter<SourcesFilterState> emit,
+  ) async {
+    emit(state.copyWith(followedSourcesStatus: SourceFilterDataLoadingStatus.loading));
+
+    final currentUser = _appBloc.state.user;
+
+    if (currentUser == null) {
+      emit(
+        state.copyWith(
+          followedSourcesStatus: SourceFilterDataLoadingStatus.failure,
+          error: const UnauthorizedException(
+            'User must be logged in to apply followed sources.',
+          ),
+        ),
+      );
+      return;
+    }
+
+    try {
+      final preferences = await _userContentPreferencesRepository.read(
+        id: currentUser.id,
+        userId: currentUser.id,
+      );
+
+      if (preferences.followedSources.isEmpty) {
+        emit(
+          state.copyWith(
+            followedSourcesStatus: SourceFilterDataLoadingStatus.success,
+            followedSources: const [],
+            error: const OperationFailedException('No followed sources found.'),
+            clearFollowedSourcesError: true,
+          ),
+        );
+        return;
+      }
+
+      emit(
+        state.copyWith(
+          followedSourcesStatus: SourceFilterDataLoadingStatus.success,
+          followedSources: preferences.followedSources,
+          finallySelectedSourceIds: preferences.followedSources.map((s) => s.id).toSet(),
+          clearFollowedSourcesError: true,
+        ),
+      );
+    } on HttpException catch (e) {
+      emit(state.copyWith(followedSourcesStatus: SourceFilterDataLoadingStatus.failure, error: e));
+    } catch (e) {
+      emit(
+        state.copyWith(
+          followedSourcesStatus: SourceFilterDataLoadingStatus.failure,
+          error: UnknownException(e.toString()),
+        ),
+      );
+    }
+  }
+
   // Helper method to filter sources based on selected countries and types
   List<Source> _getFilteredSources({
     required List<Source> allSources,