redesigned payment page + a lot of fixes

frontend/pweb/lib/models/payment/payment_state.dart

@@ -0,0 +1,27 @@
+enum PaymentState {
+  success,
+  failed,
+  cancelled,
+  processing,
+  unknown,
+}
+
+PaymentState paymentStateFromRaw(String? raw) {
+  final trimmed = (raw ?? '').trim().toUpperCase();
+  final normalized = trimmed.startsWith('PAYMENT_STATE_')
+      ? trimmed.substring('PAYMENT_STATE_'.length)
+      : trimmed;
+
+  switch (normalized) {
+    case 'SUCCESS':
+      return PaymentState.success;
+    case 'FAILED':
+      return PaymentState.failed;
+    case 'CANCELLED':
+      return PaymentState.cancelled;
+    case 'PROCESSING':
+      return PaymentState.processing;
+    default:
+      return PaymentState.unknown;
+  }
+}