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

@@ -45,7 +45,7 @@
         </Alert>
     }
 
-    <table style="width: 100%;">
+    <table class="flex-column" style="width: 100%;">
 
         <InteractionRow
             Prospect="Prospect"
@@ -148,6 +148,8 @@
         }
 
     </table>
-
-
+    
+    <div class="flex-grow-1"></div>
+    
+    <small class="text-center" style="margin-top: .5rem;">Zuletzt geändert: @Prospect?.Modified?.ToLocalTime()</small>
 </div>

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

FoodsharingSiegen.Server/Controls/ProspectContainer.razor.css

@@ -14,13 +14,15 @@
 }
 
 .pc-main {
+    display: flex;
+    flex-direction: column;
     flex-basis: 0;
     flex-grow: 1;
     max-width: 480px;
     border: 1px solid #533a20;
     border-radius: 3px;
     margin: 5px;
-    padding: 16px;
+    padding: .5rem .5rem 0 .5rem;
 }
 
 .pc-main.warning {