Изменил

Pages/AuthPage.xaml

@@ -25,6 +25,6 @@
         <Label Content="Пароль" Grid.Column="1" Grid.Row="1" VerticalAlignment="Top" Height="25" Margin="0,30,0,0"/>
         <PasswordBox Grid.Column="2" Margin="0,30,0,0" Grid.Row="1" VerticalAlignment="Top" Height="25"></PasswordBox>
         <Button Content="Вход" Grid.Column="2" Margin="0,60,0,0" Grid.Row="1" VerticalAlignment="Top"></Button>
-        <Button Content="Регистрация" Grid.Column="2" Margin="0,85,0,0" Grid.Row="1" VerticalAlignment="Top"></Button>
+        <Button Content="Регистрация" Grid.Column="2" Margin="4,95,36,0" Grid.Row="1" VerticalAlignment="Top" Grid.ColumnSpan="2"></Button>
     </Grid>
 </Page>