fix: replace StackPanel with Grid in TabControl Template to support internal ScrollViewer

[BevisQiu]

In the current ControlTemplate of TabControl, the content presenter is wrapped within a StackPanel. This causes an issue where a ScrollViewer placed inside a TabItem fails to function correctly because StackPanel provides infinite space to its children, preventing the ScrollViewer from calculating its viewport size.

This PR replaces the StackPanel with a Grid to ensure that content correctly fills the available space and allows ScrollViewer to trigger scrolling as expected.

I have tested the following XAML, and the ScrollViewer now scrolls correctly:

<TabControl>
    <TabControl.Items>
        <TabItem Header="Preview">
            <ScrollViewer 
                shadui:SmoothScrollAssist.IsEnabled="True"
                shadui:SmoothScrollAssist.BaseStepSize="70"
                shadui:SmoothScrollAssist.SmoothingFactor="20" Margin="20 20 20 0">

                <Border Height="2000">
                    <TextBlock Classes="h1" VerticalAlignment="Center" HorizontalAlignment="Center">Content</TextBlock>
                </Border>

            </ScrollViewer>
        </TabItem>

        <TabItem Header="Code">       

        </TabItem>
    </TabControl.Items>
</TabControl>