Increase visibility of a Finished Subject modal

[seanmiller26]

Discussed in #6677

^{Originally posted by seanmiller26 February 10, 2025}
To coincide with our discussion on spurious classifications we want to update our warning modal to be more noticeable.

• Add an overlay on the task area
• Add a Grommet alert icon
• Updated modal styling
• Unify the styling of the button options

At the moment, this modal only applies to projects set up with a Indexed Subject Set viewer, which includes the option to 'Choose a new subject'.

Figma specs here: https://www.figma.com/design/6rXVi6arrGTmoM08x9tpex/UI-Kit?node-id=6-1246&t=gxRoaFv2GrOnOhJp-1

Once this modal is updated, the style can be potentially be reused on this related discussion: #6675 , more decisions needed.