fix: restore auto-navigation on plate add in PlateManager

OnPlateAdded now navigates to the new plate when suppressNavigation is
false, matching the original EditNestForm behavior. Fixed CanRemoveCurrent
test to account for this auto-navigation.

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

OpenNest.Tests/PlateManagerTests.cs

@@ -390,6 +390,8 @@ public class PlateManagerTests
         Assert.False(mgr.CanRemoveCurrent);
 
         nest.CreatePlate();
+        // Auto-navigated to plate2 (empty), go back to plate1 (has parts)
+        mgr.LoadFirst();
         Assert.True(mgr.CanRemoveCurrent);
     }
 }