feat: add F key zoom-to-fit and fix middle-click conflict in PlateView

- F key triggers ZoomToFit; Ctrl+F passes through for ActionClone fill
- Middle-button double-click ZoomToFit skipped when parts are selected
  to avoid conflicting with middle-click 90° rotation

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

OpenNest/Controls/PlateView.cs

@@ -258,7 +258,7 @@ namespace OpenNest.Controls
         {
             base.OnMouseDoubleClick(e);
 
-            if (e.Button == MouseButtons.Middle)
+            if (e.Button == MouseButtons.Middle && SelectedParts.Count == 0)
                 ZoomToFit();
         }
 
@@ -270,6 +270,13 @@ namespace OpenNest.Controls
                     RemoveSelectedParts();
                     break;
 
+                case Keys.F:
+                    if ((ModifierKeys & Keys.Control) == 0)
+                        ZoomToFit();
+                    else
+                        base.OnKeyDown(e);
+                    break;
+
                 default:
                     base.OnKeyDown(e);
                     break;