intent reference generation + propagation

frontend/pweb/lib/providers/multiple_payouts.dart

@@ -188,6 +188,7 @@ class MultiplePayoutsProvider extends ChangeNotifier {
     try {
       _setState(MultiplePayoutsState.sending);
       _error = null;
+      final intentRefs = _quotedIntentRefs();
 
       final result = await payment.pay(
         metadata: <String, String>{
@@ -197,6 +198,7 @@ class MultiplePayoutsProvider extends ChangeNotifier {
           'upload_rows': _rows.length.toString(),
           ...?_uploadAmountMetadata(),
         },
+        intentRefs: intentRefs.isEmpty ? null : intentRefs,
       );
 
       _sentCount = result.length;
@@ -272,6 +274,20 @@ class MultiplePayoutsProvider extends ChangeNotifier {
   List<PaymentQuote> _quoteItems() =>
       _quotation?.quotation?.items ?? const <PaymentQuote>[];
 
+  List<String> _quotedIntentRefs() {
+    final seen = <String>{};
+    final intentRefs = <String>[];
+    for (final quote in _quoteItems()) {
+      final intentRef = (quote.intentRef ?? '').trim();
+      if (intentRef.isEmpty || seen.contains(intentRef)) {
+        continue;
+      }
+      seen.add(intentRef);
+      intentRefs.add(intentRef);
+    }
+    return intentRefs;
+  }
+
   @override
   void dispose() {
     _quotation?.removeListener(_onQuotationChanged);