Rework the document reflection into real glass

The global reflection you toggle in the Document section used to be a bespoke shader that faked a highlight and shadow overlay. It’s now the actual liquid-glass material wrapped around your icon’s silhouette, so it reacts to the light angle and bezel the same way a glass layer does.

The old Blur slider is gone; in its place you get a Highlight Width control and a linkable Opposing Highlight, matching the controls glass layers already have.