feature: support iOS login

Pixiview/Illust/IllustCollectionPage.cs

@@ -530,6 +530,10 @@ namespace Pixiview.Illust
             illustData = DoLoadIllustData(force);
             if (illustData == null)
             {
+                AppShell.Current.PushToLogin(()=>
+                {
+                    StartLoad(true);
+                });
                 //App.DebugError("illusts.load", "failed to load illusts data.");
                 IsLoading = false;
                 IsBottomLoading = false;