feat(api): Handle null user settings response gracefully

Improve error handling for user settings API call
Prevent potential null or unexpected response types
Return an empty map for new users without settings

lib/core/services/api_service.dart

@@ -1520,7 +1520,12 @@ class ApiService {
   Future<Map<String, dynamic>> getUserSettings() async {
     _traceApi('Fetching user settings');
     final response = await _dio.get('/api/v1/users/user/settings');
-    return response.data as Map<String, dynamic>;
+    final data = response.data;
+    // Handle null response from server (happens for new users with no settings)
+    if (data is Map<String, dynamic>) {
+      return data;
+    }
+    return <String, dynamic>{};
   }
 
   Future<void> updateUserSettings(Map<String, dynamic> settings) async {