fix(booking): enumerate the missing fields with commas, not repeated 'y'

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

lib/screens/user/user_map_screen.dart

@@ -214,6 +214,12 @@ class _UserMapScreenState extends State<UserMapScreen> {
   }
 
 
+  /// "a, b y c" — join(' y ') read as "a y b y c".
+  String _enumerar(List<String> items) {
+    if (items.length <= 1) return items.join();
+    return '${items.sublist(0, items.length - 1).join(', ')} y ${items.last}';
+  }
+
   /// Re-reads the professional's agenda right before creating the service.
   ///
   /// Returns true when the slot is still free, or when the agenda cannot be
@@ -596,7 +602,7 @@ class _UserMapScreenState extends State<UserMapScreen> {
                             ScaffoldMessenger.of(context).clearSnackBars();
                             ScaffoldMessenger.of(context).showSnackBar(SnackBar(
                               content: Text(
-                                  'Falta ${faltan.join(' y ')} para pedir la cita'),
+                                  'Falta ${_enumerar(faltan)} para pedir la cita'),
                             ));
                             return;
                           }