TF-460 Separate the header of MailboxDashboard on the browser

lib/features/base/mixin/app_loader_mixin.dart

@@ -0,0 +1,13 @@
+
+import 'package:core/core.dart';
+import 'package:flutter/cupertino.dart';
+
+mixin AppLoaderMixin {
+
+  Widget get loadingWidget {
+    return Center(child: SizedBox(
+        width: 24,
+        height: 24,
+        child: CupertinoActivityIndicator(color: AppColor.colorLoading)));
+  }
+}