migration to address book service

frontend/pweb/lib/utils/payment/masking.dart

@@ -0,0 +1,25 @@
+import 'dart:math';
+
+
+/// Masks a card number, leaving only the last 4 digits visible.
+/// Returns 'N/A' when the input is null or empty.
+String? maskCardNumber(String? pan) {
+  if (pan == null || pan.isEmpty) return null;
+
+  // Strip non-digits to avoid leaking formatting patterns.
+  final digits = pan.replaceAll(RegExp(r'\D'), '');
+  if (digits.length <= 4) return digits;
+
+  final last4 = digits.substring(digits.length - 4);
+  final maskedPrefix = '*' * (digits.length - 4);
+  final masked = '$maskedPrefix$last4';
+
+  // Group into blocks of 4 for readability.
+  final groups = <String>[];
+  for (var i = 0; i < masked.length; i += 4) {
+    final end = min(i + 4, masked.length);
+    groups.add(masked.substring(i, end));
+  }
+
+  return groups.join(' ');
+}