fix(router): update country filter usage in router

- Replace 'usage' parameter with 'filter' parameter in CountryFilterPage
- Change from string value 'eventCountry' to CountryFilterUsage enum
- Improve type safety and clarity in router configuration

Author: fulleni

lib/router/router.dart

@@ -546,7 +546,7 @@ GoRouter createRouter({
                               child: CountryFilterPage(
                                 title:
                                     l10n.headlinesFeedFilterEventCountryLabel,
-                                usage: 'eventCountry',
+                                filter: CountryFilterUsage.hasActiveHeadlines,
                                 key: ValueKey(initialSelection.hashCode),
                               ),
                             ),