fix: XamlParseException due to missing ThemeResource fallback

[sentry]

This PR addresses the Windows.UI.Xaml.Markup.XamlParseException that occurred when GridViewItemCheckBoxBorderBrush could not be found, leading to application crashes on certain Windows/WinUI contract versions.

The root cause was identified as the GridViewItemCheckBoxBorderBrush being a system ThemeResource that is not universally available across all Windows/WinUI contract versions where the contract13Present style is applied. The ResourceDictionary.ThemeDictionaries in Screenbox/Styles/ItemContainer.xaml lacked a fallback StaticResource definition for this key.

To fix this, StaticResource fallbacks for GridViewItemCheckBoxBorderBrush have been added to the Default, Light, and HighContrast ThemeDictionaries within Screenbox/Styles/ItemContainer.xaml:

• Default/Light themes now use ControlStrongStrokeColorDefaultBrush.
• HighContrast theme now uses SystemColorButtonTextColorBrush.

This ensures that the XAML parser can always resolve the GridViewItemCheckBoxBorderBrush resource, preventing the XamlParseException and improving application stability across different Windows versions.

Fixes SCREENBOX-32