App: Make HintContextChange() synchronous by using MainDispatcher

Elwig/Services/MemberService.cs

@@ -338,7 +338,7 @@ namespace Elwig.Services {
                 var b = new Billing(year);
                 await b.FinishSeason();
                 await b.CalculateBuckets();
-                await App.HintContextChange();
+                App.HintContextChange();
 
                 using var ctx = new AppDbContext();
                 var data = await DeliveryConfirmationDeliveryData.ForMember(ctx.DeliveryParts, year, m);
@@ -626,7 +626,7 @@ namespace Elwig.Services {
                 }
             }
 
-            await App.HintContextChange();
+            App.HintContextChange();
 
             return newMgNr;
         }