[chore:] implemented csv-export access from gui

MainWindow.axaml.cs

@@ -424,8 +424,22 @@ public partial class MainWindow : Window
         } catch {}
     }
 
-    private void BtnExportCourseCSV_OnClick(object? sender, RoutedEventArgs e)
+    private async void BtnExportCourseCSV_OnClick(object? sender, RoutedEventArgs e)
     {
-        // export as csv here
+        var topLevel = GetTopLevel(this);
+        var file = await topLevel!.StorageProvider.SaveFilePickerAsync(new FilePickerSaveOptions
+        {
+            Title = "CSV-Datei speichern",
+            SuggestedFileType = new FilePickerFileType(".csv-Datei")
+            {
+                Patterns = new[] { "*.csv" }
+            }
+            
+        });
+
+        if (file == null) return;
+
+        ExportUtility.ExportToCSV(file.Path.AbsolutePath);
+
     }
 }