[upstream #16461] Theme-owned dialog scrim opacity, and whether a scrim may lift the page #181

Click UI's dialog scrim is click.dialog.color.opaqueBackground: lch(6.7738 0 none / 0.75) in light and lch(40.731 0 none / 0.75) (about #606060) in dark. LibreChat carries the scrim alpha in each utility (bg-surface-overlay/80, /65, /90, /50, /30), so a theme cannot set it, and the dark gray would leave the dialog surface about 2.1:1 against the scrim, under the 3:1 boundary dialog-scrim-role.spec.ts holds. The ClickHouse theme keeps black in dark mode and records it as an unsourced value in clickui.spec.ts.

Gap: a scrim opacity role, and a decision on whether a theme may lift the page instead of dimming it when the dialog has its own border and elevation.