Issue #3624494: Gin cuts a toolbar icon out of a mask, so the Booking item is the empty square the stylesheet exists to prevent

Closes #3624494.

Gin does not paint the picture core's toolbar paints. It fills the item's ::before with a flat color and cuts it to shape with mask-image: var(--icon), defaulting that variable to an empty rounded square for every item it has no drawing of its own for, and its background shorthand drops the background-image this stylesheet sets. Both halves reach the Booking item, so under Gin it was the one item in the bar with nothing in it.

The drawing is now held in one custom property and handed to both: to background-image for core, which paints the picture and takes its color from it, and to --icon for Gin, which reads only the alpha and paints the color itself. One variable rather than a second copy of the URL-encoded SVG, because two copies drift the first time one of them is touched and nothing about a toolbar fails when they do. Hover, active and dark mode stay Gin's to paint, so the mask needs no is-active twin.

ToolbarIconTest gains one case: the item hands --icon a variable, background-image is handed the same one, and that variable is where the drawing is written down. It fails against the stylesheet as it was.