fix：第一轮跑

lib/features/web/application/mall_web_host_controller.dart

@@ -0,0 +1,30 @@
+import 'package:flutter/foundation.dart';
+
+class MallWebHostState {
+  const MallWebHostState({this.url, this.visible = false});
+  final String? url;
+  final bool visible;
+}
+
+/// 进程常驻的商城 WebView 状态，切换原生 Tab 时不销毁商城页面。
+class MallWebHostController {
+  MallWebHostController._();
+  static final instance = MallWebHostController._();
+  final state = ValueNotifier(const MallWebHostState());
+  Future<void> Function()? _backHandler;
+
+  void preload(String url) {
+    final old = state.value;
+    if (old.url == url) return;
+    state.value = MallWebHostState(url: url, visible: old.visible);
+  }
+
+  void open(String url) =>
+      state.value = MallWebHostState(url: url, visible: true);
+  void dismiss() => state.value = MallWebHostState(url: state.value.url);
+  void clear() => state.value = const MallWebHostState();
+  void attachBackHandler(Future<void> Function() handler) =>
+      _backHandler = handler;
+  void detachBackHandler() => _backHandler = null;
+  Future<void> handleBack() => _backHandler?.call() ?? Future.value();
+}