Fix and harden prospect creation against Blazor runtime crashes

- Handle empty numeric input safely with nullable integer binding.

- Add semaphore locks to prevent double-click invocation errors on mobile.

- Implement global exception handling and user notifications for prospect operations.

FoodsharingSiegen.Server/Dialogs/EditProspectDialog.razor.cs

@@ -96,16 +96,34 @@ namespace FoodsharingSiegen.Server.Dialogs
                 if (IsUpdateMode)
                 {
                     var updateR = await ProspectService.UpdateAsync(Prospect);
-                    if (updateR.Success && OnSuccess != null) await OnSuccess.Invoke();
+                    if (updateR.Success)
+                    {
+                        if (OnSuccess != null) await OnSuccess.Invoke();
+                        await ModalService.Hide();
+                    }
+                    else
+                    {
+                        await Notification.Error(updateR.Exception?.Message ?? "Unbekannter Fehler beim Speichern.", "Fehler");
+                    }
                 }
                 else
                 {
                     var addR = await ProspectService.AddProspectAsync(Prospect);
-                    if (addR.Success && OnSuccess != null) await OnSuccess.Invoke();
-                }
-
+                    if (addR.Success)
+                    {
+                        if (OnSuccess != null) await OnSuccess.Invoke();
                         await ModalService.Hide();
                     }
+                    else
+                    {
+                        await Notification.Error(addR.Exception?.Message ?? "Unbekannter Fehler beim Hinzufügen.", "Fehler");
+                    }
+                }
+            }
+            catch (Exception ex)
+            {
+                await Notification.Error(ex.Message, "Systemfehler");
+            }
             finally
             {
                 _isSaving = false;

FoodsharingSiegen.Server/Pages/Prospects.razor.cs

@@ -99,6 +99,8 @@ namespace FoodsharingSiegen.Server.Pages
         ///     Loads the prospects (a. beging, 11.04.2022)
         /// </summary>
         private async Task LoadProspects()
+        {
+            try
             {
                 var parameter = new GetProspectsParameter
                 {
@@ -110,6 +112,11 @@ namespace FoodsharingSiegen.Server.Pages
 
                 await InvokeAsync(StateHasChanged);
             }
+            catch (Exception ex)
+            {
+                await Notification.Error(ex.Message, "Fehler beim Laden");
+            }
+        }
 
         #endregion
     }