refactor(headlines-feed): assume logged-in user for content preferences

- Remove null checks for current user in countries, sources, and topics filter BLOCS
- Simplify user authentication assumption in headlines filter page
- Update user-related operations to directly access user properties

Author: fulleni

lib/headlines-feed/bloc/countries_filter_bloc.dart

@@ -98,19 +98,7 @@ class CountriesFilterBloc
       state.copyWith(followedCountriesStatus: CountriesFilterStatus.loading),
     );
 
-    final currentUser = _appBloc.state.user;
-
-    if (currentUser == null) {
-      emit(
-        state.copyWith(
-          followedCountriesStatus: CountriesFilterStatus.failure,
-          error: const UnauthorizedException(
-            'User must be logged in to apply followed countries.',
-          ),
-        ),
-      );
-      return;
-    }
+    final currentUser = _appBloc.state.user!;
 
     try {
       final preferences = await _userContentPreferencesRepository.read(

lib/headlines-feed/bloc/sources_filter_bloc.dart

@@ -204,19 +204,7 @@ class SourcesFilterBloc extends Bloc<SourcesFilterEvent, SourcesFilterState> {
       ),
     );
 
-    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;
-    }
+    final currentUser = _appBloc.state.user!;
 
     try {
       final preferences = await _userContentPreferencesRepository.read(

lib/headlines-feed/bloc/topics_filter_bloc.dart

@@ -126,19 +126,7 @@ class TopicsFilterBloc extends Bloc<TopicsFilterEvent, TopicsFilterState> {
   ) async {
     emit(state.copyWith(followedTopicsStatus: TopicsFilterStatus.loading));
 
-    final currentUser = _appBloc.state.user;
-
-    if (currentUser == null) {
-      emit(
-        state.copyWith(
-          followedTopicsStatus: TopicsFilterStatus.failure,
-          error: const UnauthorizedException(
-            'User must be logged in to apply followed topics.',
-          ),
-        ),
-      );
-      return;
-    }
+    final currentUser = _appBloc.state.user!;
 
     try {
       final preferences = await _userContentPreferencesRepository.read(

lib/headlines-feed/view/headlines_filter_page.dart

@@ -88,18 +88,7 @@ class _HeadlinesFilterPageState extends State<HeadlinesFilterPage> {
     });
 
     final appState = context.read<AppBloc>().state;
-    final currentUser = appState.user;
-
-    if (currentUser == null) {
-      setState(() {
-        _isLoadingFollowedFilters = false;
-        _useFollowedFilters = false;
-        _loadFollowedFiltersError = AppLocalizationsX(
-          context,
-        ).l10n.mustBeLoggedInToUseFeatureError;
-      });
-      return;
-    }
+    final currentUser = appState.user!;
 
     try {
       final preferencesRepo = context