fix(headlines-feed): update country filter criteria for source selection

- Change country filter from 'usage': 'headquarters' to 'usage': 'hasActiveHeadlines'
- This modification ensures that only countries with active headlines are considered when filtering sources

Author: fulleni

lib/headlines-feed/bloc/sources_filter_bloc.dart

@@ -46,7 +46,7 @@ class SourcesFilterBloc extends Bloc<SourcesFilterEvent, SourcesFilterState> {
     );
     try {
       final availableCountries = (await _countriesRepository.readAll(
-        filter: {'usage': 'headquarters'},
+        filter: {'usage': 'hasActiveHeadlines'},
       )).items;
       final initialSelectedSourceIds = event.initialSelectedSources
           .map((s) => s.id)