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

frontend/app/lib/destinations/list.dart

@@ -0,0 +1,165 @@
+import 'package:flutter/material.dart';
+
+import 'package:provider/provider.dart';
+
+import 'package:sxchnge_wallet/sxchnge_wallet.dart';
+
+import 'copy.dart';
+import 'detail.dart';
+import 'form.dart';
+
+import '../generated/i18n/app_localizations.dart';
+
+class DestinationsPage extends StatefulWidget {
+  const DestinationsPage({super.key, this.payoutContext});
+  final PayoutContext? payoutContext;
+  @override
+  State<DestinationsPage> createState() => _DestinationsPageState();
+}
+
+class _DestinationsPageState extends State<DestinationsPage> {
+  String query = '';
+  late final selection = DestinationSelectionProvider(
+    context.read<DestinationsProvider>().service,
+    widget.payoutContext ?? const PayoutContext(''),
+  );
+  Future<void> refresh() async {
+    final p = context.read<DestinationsProvider>();
+    await p.refresh();
+    if (!mounted) return;
+    await selection.assess(p.methods);
+  }
+
+  @override
+  void initState() {
+    super.initState();
+    WidgetsBinding.instance.addPostFrameCallback((_) {
+      if (mounted) refresh();
+    });
+  }
+
+  Future<void> add() async {
+    final result = await Navigator.push<DestinationInput>(
+      context,
+      MaterialPageRoute(
+        builder: (_) =>
+            DestinationFormPage(payoutContext: widget.payoutContext),
+      ),
+    );
+    if (!mounted) return;
+    if (result != null && widget.payoutContext != null) {
+      Navigator.pop(context, result);
+    } else {
+      await refresh();
+    }
+  }
+
+  Future<void> choose(SavedDestination d) async {
+    if (widget.payoutContext == null) {
+      await Navigator.push<void>(
+        context,
+        MaterialPageRoute(
+          builder: (_) => DestinationDetailPage(destination: d),
+        ),
+      );
+      if (mounted) await refresh();
+    } else {
+      final ref = await selection.select(d);
+      if (mounted && ref != null) Navigator.pop(context, ref);
+    }
+  }
+
+  @override
+  void dispose() {
+    selection.dispose();
+    super.dispose();
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    final l = AppLocalizations.of(context)!;
+    final p = context.watch<DestinationsProvider>();
+    return Scaffold(
+      appBar: AppBar(
+        title: Text(widget.payoutContext == null ? l.dTitle : l.dChoose),
+        actions: [
+          IconButton(
+            tooltip: l.dReload,
+            onPressed: p.loading ? null : refresh,
+            icon: const Icon(Icons.refresh),
+          ),
+        ],
+      ),
+      body: ListenableBuilder(
+        listenable: selection,
+        builder: (context, _) => ListView(
+          padding: const EdgeInsets.all(24),
+          children: [
+            if (p.capabilities?.simulated == true) ...[
+              Text(l.dDemo),
+              const SizedBox(height: 16),
+            ],
+            FilledButton.icon(
+              onPressed: p.loading || (p.capabilities?.types.isEmpty ?? true)
+                  ? null
+                  : add,
+              icon: const Icon(Icons.add),
+              label: Text(widget.payoutContext == null ? l.dAdd : l.dNew),
+            ),
+            if (p.capabilities?.types.isEmpty == true)
+              Text(l.dManagementUnavailable),
+            const SizedBox(height: 20),
+            TextField(
+              decoration: InputDecoration(
+                labelText: l.dSearch,
+                prefixIcon: const Icon(Icons.search),
+              ),
+              onChanged: (v) => setState(() => query = v),
+            ),
+            if (p.loading || selection.busy) const LinearProgressIndicator(),
+            if (p.failed) Text(l.dUnavailable),
+            if (selection.error != null)
+              Semantics(
+                liveRegion: true,
+                child: Text(destinationIssue(l, selection.error!)),
+              ),
+            if (widget.payoutContext != null &&
+                !p.loading &&
+                p.methods.isNotEmpty &&
+                !selection.assessments.values.any((a) => a.eligible))
+              Text(l.dNoEligible),
+            for (final type
+                in p.capabilities?.types ?? <DestinationType>[]) ...[
+              const SizedBox(height: 24),
+              Text(
+                destinationType(l, type),
+                style: Theme.of(context).textTheme.titleLarge,
+              ),
+              const SizedBox(height: 8),
+              if (p.search(query, type).isEmpty && !p.loading && !p.failed)
+                Text(query.isEmpty ? l.dEmpty : l.dNoMatches),
+              for (final d in p.search(query, type))
+                Card(
+                  child: ListTile(
+                    contentPadding: const EdgeInsets.all(16),
+                    title: Text(d.label),
+                    subtitle: Column(
+                      crossAxisAlignment: CrossAxisAlignment.start,
+                      children: [
+                        if (d.nickname.isNotEmpty) Text(d.details.display),
+                        if (selection.assessments[d.id]
+                            case final PayoutEligibility e)
+                          Text(eligibilityText(l, e)),
+                      ],
+                    ),
+                    trailing: const Icon(Icons.chevron_right),
+                    onTap: selection.busy ? null : () => choose(d),
+                  ),
+                ),
+            ],
+          ],
+        ),
+      ),
+    );
+  }
+}