LightBox: Use a Dialog component variant

[elstoc]

Firstly, preventing the underlying gallery from scrolling is important (and dialog does this) -- this looks to have been broken by moving to layout rendering. Also dialog has some a11y and animating that would be useful to leverage

[elstoc]

Looks like dialog has some keypress suppression so it might be better to just improve what's already done (though modal=false might mitigate this)