feat: Update default API endpoint to production URL in ApiConfig and settings screen

lib/data/api_client.dart

@@ -15,7 +15,7 @@ class ApiConfig {
   static const String _endpointKey = 'api_endpoint_v1';
 
   /// Default endpoint for local development. Can be overridden by user.
-  static const String defaultEndpoint = 'http://localhost:3000/api';
+  static const String defaultEndpoint = 'https://notas-api.lpncnd.es/api';
 
   static Future<String> getEndpoint() async {
     final prefs = await SharedPreferences.getInstance();

lib/screens/settings_screen.dart

@@ -524,7 +524,7 @@ class _SettingsScreenState extends State<SettingsScreen> {
                         ],
                       ),
                       const SizedBox(height: 24),
-                      const Text('API endpoint (ej: http://localhost:3000/api)'),
+                      const Text('API endpoint (ej: https://notas-api.lpncnd.es/api)'),
                       const SizedBox(height: 8),
                       _buildResponsiveInputActionsRow(
                         input: _endpointLoading