create promocode page

lib/presentation/state/promo_code_state.dart

@@ -0,0 +1,25 @@
+enum PromoCodeStatus { initial, loading, success, failure }
+
+class PromoCodeState {
+  final PromoCodeStatus status;
+  final double? newBalance;
+  final String? errorMessage;
+
+  const PromoCodeState({
+    this.status = PromoCodeStatus.initial,
+    this.newBalance,
+    this.errorMessage,
+  });
+
+  PromoCodeState copyWith({
+    PromoCodeStatus? status,
+    double? newBalance,
+    String? errorMessage,
+  }) {
+    return PromoCodeState(
+      status: status ?? this.status,
+      newBalance: newBalance ?? this.newBalance,
+      errorMessage: errorMessage ?? this.errorMessage,
+    );
+  }
+}