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

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

@@ -0,0 +1,65 @@
+import 'package:flutter/foundation.dart';
+
+import 'models.dart';
+import 'service.dart';
+
+class DestinationsProvider extends ChangeNotifier {
+  DestinationsProvider(this.service);
+  final DestinationService service;
+  DestinationCapabilities? capabilities;
+  List<SavedDestination> methods = [];
+  bool loading = false, failed = false, _disposed = false;
+  int _revision = 0;
+  Future<void> refresh() async {
+    final revision = ++_revision;
+    loading = true;
+    failed = false;
+    notifyListeners();
+    try {
+      final caps = await service.capabilities();
+      final rows = await service.list();
+      if (_disposed || revision != _revision) return;
+      capabilities = caps;
+      methods = rows;
+    } catch (_) {
+      if (_disposed || revision != _revision) return;
+      failed = true;
+    }
+    if (_disposed) return;
+    loading = false;
+    notifyListeners();
+  }
+
+  List<SavedDestination> search(String query, DestinationType type) {
+    final q = query.trim().toLowerCase();
+    final rows = methods
+        .where(
+          (d) =>
+              d.details.type == type &&
+              '${d.label} ${d.details.display}'.toLowerCase().contains(q),
+        )
+        .toList();
+    rows.sort(
+      (a, b) => (b.lastUsed ?? DateTime(1970)).compareTo(
+        a.lastUsed ?? DateTime(1970),
+      ),
+    );
+    return rows;
+  }
+
+  void reset() {
+    _revision++;
+    methods = [];
+    capabilities = null;
+    loading = failed = false;
+    service.clearTransient();
+    notifyListeners();
+  }
+
+  @override
+  void dispose() {
+    _disposed = true;
+    _revision++;
+    super.dispose();
+  }
+}