Add saved payout destination UX and service contracts (#54)

frontend/packages/sxchnge_wallet/lib/src/destinations/selection.dart

@@ -0,0 +1,64 @@
+import 'package:flutter/foundation.dart';
+
+import 'models.dart';
+import 'service.dart';
+
+class DestinationSelectionProvider extends ChangeNotifier {
+  DestinationSelectionProvider(this.service, this.context);
+  final DestinationService service;
+  final PayoutContext context;
+  final assessments = <String, PayoutEligibility>{};
+  bool busy = false, _disposed = false;
+  DestinationIssue? error;
+  Future<void> assess(List<SavedDestination> methods) async {
+    for (final d in methods) {
+      try {
+        final assessment = await service.assess(d.input, context);
+        if (_disposed) return;
+        assessments[d.id] = assessment;
+      } on DestinationException catch (e) {
+        if (_disposed) return;
+        assessments[d.id] = PayoutEligibility(
+          EligibilityState.requiresAction,
+          reason: e.issue,
+        );
+      } catch (_) {
+        if (_disposed) return;
+        assessments[d.id] = const PayoutEligibility(
+          EligibilityState.temporarilyUnavailable,
+          reason: DestinationIssue.unavailable,
+        );
+      }
+    }
+    if (!_disposed) notifyListeners();
+  }
+
+  Future<SavedReference?> select(SavedDestination d) async {
+    if (busy) return null;
+    busy = true;
+    error = null;
+    notifyListeners();
+    SavedReference? result;
+    try {
+      final current = await service.get(d.id);
+      final a = await service.assess(current.input, context);
+      if (_disposed) return null;
+      assessments[d.id] = a;
+      if (a.eligible) result = current.input;
+    } on DestinationException catch (e) {
+      error = e.issue;
+    } catch (_) {
+      error = DestinationIssue.unavailable;
+    }
+    if (_disposed) return null;
+    busy = false;
+    notifyListeners();
+    return result;
+  }
+
+  @override
+  void dispose() {
+    _disposed = true;
+    super.dispose();
+  }
+}