style(account): use theme-aware icon color for unfollow actions

- Replace hardcoded red color with theme's error color for unfollow icons
- Remove 'const' keyword from Icon widgets to allow theme color updates
- Update made across multiple list pages: countries, sources, and topics

Author: fulleni

lib/account/view/manage_followed_items/countries/followed_countries_list_page.dart

@@ -85,9 +85,9 @@ class FollowedCountriesListPage extends StatelessWidget {
                 ),
                 title: Text(country.name),
                 trailing: IconButton(
-                  icon: const Icon(
+                  icon: Icon(
                     Icons.remove_circle_outline,
-                    color: Colors.red,
+                    color: Theme.of(context).colorScheme.error,
                   ),
                   tooltip: l10n.unfollowCountryTooltip(country.name),
                   onPressed: () {

lib/account/view/manage_followed_items/sources/followed_sources_list_page.dart

@@ -82,9 +82,9 @@ class FollowedSourcesListPage extends StatelessWidget {
                   overflow: TextOverflow.ellipsis,
                 ),
                 trailing: IconButton(
-                  icon: const Icon(
+                  icon: Icon(
                     Icons.remove_circle_outline,
-                    color: Colors.red,
+                    color: Theme.of(context).colorScheme.error,
                   ),
                   tooltip: l10n.unfollowSourceTooltip(source.name),
                   onPressed: () {

lib/account/view/manage_followed_items/topics/followed_topics_list_page.dart

@@ -90,9 +90,9 @@ class FollowedTopicsListPage extends StatelessWidget {
                   overflow: TextOverflow.ellipsis,
                 ),
                 trailing: IconButton(
-                  icon: const Icon(
+                  icon: Icon(
                     Icons.remove_circle_outline,
-                    color: Colors.red,
+                    color: Theme.of(context).colorScheme.error,
                   ),
                   tooltip: l10n.unfollowTopicTooltip(topic.name),
                   onPressed: () {