feat: integrate unified payment and wallet refunds

apps/user_app/lib/data/services/payment_jsapi.dart

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+export 'payment_jsapi_stub.dart'
+    if (dart.library.js_interop) 'payment_jsapi_web.dart';

apps/user_app/lib/data/services/payment_jsapi_stub.dart

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+Future<void> invokeWechatJsapi(String clientArgs) =>
+    throw UnsupportedError('微信 JSAPI 仅支持 Web');

apps/user_app/lib/data/services/payment_jsapi_web.dart

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+import 'dart:async';
+import 'dart:convert';
+import 'dart:js_interop';
+
+@JS('WeixinJSBridge.invoke')
+external void _invoke(String name, JSAny? arguments, JSFunction callback);
+
+Future<void> invokeWechatJsapi(String clientArgs) {
+  final completer = Completer<void>();
+  final arguments = jsonDecode(clientArgs).jsify();
+  _invoke(
+    'getBrandWCPayRequest',
+    arguments,
+    (JSAny? rawResult) {
+      final result = rawResult.dartify();
+      final message = result is Map ? result['err_msg']?.toString() ?? '' : '';
+      if (message == 'get_brand_wcpay_request:ok') {
+        completer.complete();
+      } else {
+        completer.completeError(StateError('微信支付未完成：$message'));
+      }
+    }.toJS,
+  );
+  return completer.future;
+}

apps/user_app/lib/data/services/payment_launcher.dart

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+import 'dart:convert';
+
+import 'package:flutter/foundation.dart';
+import 'package:fluwx/fluwx.dart';
+import 'package:tobias/tobias.dart';
+import 'package:url_launcher/url_launcher.dart';
+
+import 'payment_jsapi.dart';
+
+/// 只负责调起支付渠道；支付结果必须重新向服务端查询确认。
+class PaymentLauncher {
+  PaymentLauncher({Fluwx? wechat, Tobias? alipay})
+      : _wechat = wechat ?? Fluwx(),
+        _alipay = alipay ?? Tobias();
+
+  final Fluwx _wechat;
+  final Tobias _alipay;
+
+  Future<void> launch(Map<String, Object?> payment) async {
+    final channel = payment['channel'];
+    final payType = payment['pay_type'];
+    final argsText = payment['client_args'] as String? ?? '';
+
+    // 支付宝手机网站支付返回可直接打开的收银台 URL。
+    if (payType == 'wap') {
+      final uri = Uri.tryParse(argsText);
+      if (uri == null ||
+          !await launchUrl(uri, mode: LaunchMode.externalApplication)) {
+        throw StateError('无法打开支付页面');
+      }
+      return;
+    }
+    if (kIsWeb && payType == 'jsapi') {
+      await invokeWechatJsapi(argsText);
+      return;
+    }
+    if (channel == 'alipay') {
+      await _alipay.pay(argsText);
+      return;
+    }
+
+    final args = (jsonDecode(argsText) as Map).cast<String, dynamic>();
+    final accepted = await _wechat.pay(
+      which: Payment(
+        appId: payment['app_id'] as String? ?? '',
+        partnerId: args['partnerId'] as String,
+        prepayId: args['prepayId'] as String,
+        packageValue: args['package'] as String,
+        nonceStr: args['nonceStr'] as String,
+        timestamp: int.parse(args['timeStamp'] as String),
+        sign: args['sign'] as String,
+      ),
+    );
+    if (!accepted) throw StateError('微信未接受支付请求');
+  }
+}