Fix back button from baseline leading to black screen

Changed pushReplacement to push in onboarding navigation so the
onboarding screen stays in the nav stack when moving to baseline.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

lib/screens/onboarding_screen.dart

@@ -258,7 +258,7 @@ class _OnboardingScreenState extends State<OnboardingScreen> {
   }
 
   void _navigateToBaseline() {
-    Navigator.of(context).pushReplacement(
+    Navigator.of(context).push(
       MaterialPageRoute(builder: (_) => const BaselineScreen()),
     );
   }