fix: auto-select first part and refresh quantity in CadConverter file list

Fire SelectedIndexChanged when the first item is added so the preview
loads automatically. Invalidate the file list after quantity changes so
the badge repaints immediately.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

OpenNest/Controls/FileListControl.cs

@@ -63,7 +63,11 @@ namespace OpenNest.Controls
         public void AddItem(FileListItem item)
         {
             items.Add(item);
-            if (items.Count == 1) selectedIndex = 0;
+            if (items.Count == 1)
+            {
+                selectedIndex = 0;
+                SelectedIndexChanged?.Invoke(this, selectedIndex);
+            }
             Invalidate();
         }
 

OpenNest/Forms/CadConverterForm.cs

@@ -194,8 +194,10 @@ namespace OpenNest.Forms
         private void OnQuantityChanged(object sender, EventArgs e)
         {
             var item = CurrentItem;
-            if (item != null)
-                item.Quantity = (int)numQuantity.Value;
+            if (item == null) return;
+
+            item.Quantity = (int)numQuantity.Value;
+            fileList.Invalidate();
         }
 
         private void OnCustomerChanged(object sender, EventArgs e)