feat: gate News tab and briefing news cards on server rss_enabled setting

Fetch server-side settings on app launch via new serverSettingsProvider.
Hide News from bottom sheet and NavigationRail when RSS is disabled.
Skip news card rendering in briefing messages when disabled.
Refactor tab navigation to use dynamic tabs list and route-based refresh.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

lib/data/api/settings_api.dart

@@ -10,4 +10,13 @@ class SettingsApi {
       data: {'user_timezone': ianaTimezone},
     );
   }
+
+  Future<Map<String, dynamic>> getAll() async {
+    final response = await _dio.get<Map<String, dynamic>>('/api/settings');
+    return response.data ?? {};
+  }
+
+  Future<void> update(Map<String, String> updates) async {
+    await _dio.put<void>('/api/settings', data: updates);
+  }
 }