Add MainService for state management and update Index and AuthDialog to utilize it

src/FrontOffice.Main/Utilities/MainService.cs

@@ -0,0 +1,13 @@
+using Microsoft.AspNetCore.Components;
+
+namespace FrontOffice.Main.Utilities;
+
+public class MainService
+{
+    public event Action OnChangeHandler;
+
+    public virtual void OnOnChangeHandler()
+    {
+        OnChangeHandler?.Invoke();
+    }
+}