Add 'Modified' tracking for Prospect entity

Introduce a 'Modified' field to track the last modification date of prospects. Automatically update this field on changes and integrate it in UI display for better transparency.

FoodsharingSiegen.Server/Controls/ProspectContainer.razor.cs

@@ -72,7 +72,7 @@ namespace FoodsharingSiegen.Server.Controls
 
         private async Task EditProspectAsync()
         {
-            await EditProspectDialog.ShowAsync(ModalService, () => InvokeAsync(StateHasChanged), Prospect);
+            await EditProspectDialog.ShowAsync(ModalService, OnDataChanged ?? (async () => await Task.CompletedTask), Prospect);
         }
 
         #endregion