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

frontend/app/lib/destinations/crypto.dart

@@ -0,0 +1,128 @@
+import 'package:flutter/material.dart';
+
+import 'package:sxchnge_wallet/sxchnge_wallet.dart';
+
+import '../generated/i18n/app_localizations.dart';
+
+class CryptoFields extends StatefulWidget {
+  const CryptoFields({
+    super.key,
+    required this.capabilities,
+    required this.onChanged,
+    this.initial,
+    this.errorField,
+  });
+  final DestinationCapabilities capabilities;
+  final ValueChanged<DestinationDetails?> onChanged;
+  final CryptoDestination? initial;
+  final String? errorField;
+  @override
+  State<CryptoFields> createState() => _CryptoFieldsState();
+}
+
+class _CryptoFieldsState extends State<CryptoFields> {
+  late final address = TextEditingController(text: widget.initial?.address);
+  late final memo = TextEditingController(text: widget.initial?.memo);
+  late String? assetId = widget.initial?.assetId;
+  late String? networkId = widget.initial?.networkId;
+  void changed() {
+    final a = widget.capabilities.assets
+        .where((a) => a.id == assetId && a.networkId == networkId)
+        .firstOrNull;
+    widget.onChanged(
+      a == null
+          ? null
+          : CryptoDestination(
+              assetId: a.id,
+              networkId: a.networkId,
+              address: address.text,
+              memo: memo.text,
+              assetLabel: a.label,
+              networkLabel: a.networkLabel,
+            ),
+    );
+  }
+
+  @override
+  void dispose() {
+    address.dispose();
+    memo.dispose();
+    super.dispose();
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    final l = AppLocalizations.of(context)!;
+    final assets = widget.capabilities.assets;
+    final selected = assets
+        .where((a) => a.id == assetId && a.networkId == networkId)
+        .firstOrNull;
+    return Column(
+      crossAxisAlignment: CrossAxisAlignment.stretch,
+      children: [
+        DropdownButtonFormField<String>(
+          key: ValueKey('asset-$assetId'),
+          initialValue: assets.any((a) => a.id == assetId) ? assetId : null,
+          isExpanded: true,
+          decoration: InputDecoration(labelText: l.dAsset),
+          items: [
+            for (final a in assets)
+              DropdownMenuItem(value: a.id, child: Text(a.label)),
+          ],
+          onChanged: (v) {
+            setState(() {
+              assetId = v;
+              networkId = null;
+              memo.clear();
+            });
+            changed();
+          },
+        ),
+        const SizedBox(height: 16),
+        DropdownButtonFormField<String>(
+          key: ValueKey('network-$assetId-$networkId'),
+          initialValue:
+              assets.any((a) => a.id == assetId && a.networkId == networkId)
+              ? networkId
+              : null,
+          isExpanded: true,
+          decoration: InputDecoration(labelText: l.dNetwork),
+          items: [
+            for (final a in assets.where((a) => a.id == assetId))
+              DropdownMenuItem(value: a.networkId, child: Text(a.networkLabel)),
+          ],
+          onChanged: (v) {
+            setState(() => networkId = v);
+            changed();
+          },
+        ),
+        const SizedBox(height: 16),
+        TextField(
+          key: const ValueKey('destination-address'),
+          controller: address,
+          autocorrect: false,
+          enableSuggestions: false,
+          decoration: InputDecoration(
+            labelText: l.dAddress,
+            errorText: widget.errorField == 'address' ? l.dInvalid : null,
+          ),
+          onChanged: (_) => changed(),
+        ),
+        if (selected?.memoSupported == true) ...[
+          const SizedBox(height: 16),
+          TextField(
+            key: const ValueKey('destination-memo'),
+            controller: memo,
+            decoration: InputDecoration(
+              labelText: l.dMemo,
+              errorText: widget.errorField == 'memo' ? l.dInvalid : null,
+            ),
+            onChanged: (_) => changed(),
+          ),
+        ],
+        const SizedBox(height: 12),
+        Text(l.dCryptoHelp),
+      ],
+    );
+  }
+}