small fixes for single payout and big chunck for multiple payouts

frontend/pweb/lib/services/payments/csv_input.dart

@@ -0,0 +1,53 @@
+import 'dart:async';
+
+import 'package:universal_html/html.dart' as html;
+
+
+class PickedCsvFile {
+  final String name;
+  final String content;
+
+  const PickedCsvFile({required this.name, required this.content});
+}
+
+abstract class CsvInputService {
+  Future<PickedCsvFile?> pickCsv();
+}
+
+class WebCsvInputService implements CsvInputService {
+  @override
+  Future<PickedCsvFile?> pickCsv() async {
+    final input = html.FileUploadInputElement()
+      ..accept = '.csv,text/csv'
+      ..multiple = false;
+
+    final completer = Completer<html.File?>();
+    input.onChange.listen((_) {
+      completer.complete(
+        input.files?.isNotEmpty == true ? input.files!.first : null,
+      );
+    });
+    input.click();
+
+    final file = await completer.future;
+    if (file == null) return null;
+
+    final reader = html.FileReader();
+    final readCompleter = Completer<String>();
+    reader.onError.listen((_) {
+      readCompleter.completeError(StateError('Failed to read file'));
+    });
+    reader.onLoadEnd.listen((_) {
+      final result = reader.result;
+      if (result is String) {
+        readCompleter.complete(result);
+      } else {
+        readCompleter.completeError(StateError('Unsupported file payload'));
+      }
+    });
+    reader.readAsText(file);
+
+    final content = await readCompleter.future;
+    return PickedCsvFile(name: file.name, content: content);
+  }
+}