docs(content_management): explain background fetching for countries

- Add detailed comments in both create_headline_bloc.dart and edit_headline_bloc.dart
- Explain the reason for using background fetching for countries
- Highlight UI consistency and technical limitations as motivations
- Describe how the UI updates progressively in the background

Author: fulleni

lib/content_management/bloc/create_headline/create_headline_bloc.dart

@@ -76,7 +76,16 @@ class CreateHeadlineBloc
         ),
       );
 
-      // Start background fetching for all countries
+      // After the initial page of countries is loaded, start a background
+      // process to fetch all remaining pages.
+      //
+      // This approach is used for the following reasons:
+      // 1. UI Consistency: It allows us to use the standard
+      //    `DropdownButtonFormField`, which is used elsewhere in the app.
+      // 2. Technical Limitation: The standard dropdown does not expose a
+      //
+      // The UI will update progressively and silently in the background as
+      // more data arrives.
       while (state.countriesHasMore) {
         final nextCountries = await _countriesRepository.readAll(
           pagination: PaginationOptions(cursor: state.countriesCursor),

lib/content_management/bloc/edit_headline/edit_headline_bloc.dart

@@ -87,7 +87,16 @@ class EditHeadlineBloc extends Bloc<EditHeadlineEvent, EditHeadlineState> {
         ),
       );
 
-      // Start background fetching for all countries
+      // After the initial page of countries is loaded, start a background
+      // process to fetch all remaining pages.
+      //
+      // This approach is used for the following reasons:
+      // 1. UI Consistency: It allows us to use the standard
+      //    `DropdownButtonFormField`, which is used elsewhere in the app.
+      // 2. Technical Limitation: The standard dropdown does not expose a
+      //
+      // The UI will update progressively and silently in the background as
+      // more data arrives.
       while (state.countriesHasMore) {
         final nextCountries = await _countriesRepository.readAll(
           pagination: PaginationOptions(cursor: state.countriesCursor),