feat: clear navigation stack after successful authentication in login and signup flows

lib/pages/signup.dart

@@ -128,7 +128,11 @@ class _SignUpPageState extends State<SignUpPage> {
                 backgroundColor: Colors.green,
               ),
             );
-            Navigator.pushReplacementNamed(context, '/home');
+            Navigator.pushNamedAndRemoveUntil(
+              context,
+              '/home',
+              (route) => false,
+            );
           } else if (state is AuthError) {
             _errorService.showError(message: state.message);
           }