[upstream #16492] Mobile drawer scrim and edge on the theme scrim role (after #16248) #199

Split from LibreChat-AI#16492, which adds the dialog scrim opacity roles (scrimOpacity, alertScrimOpacity, modalScrimOpacity, exposed as bg-scrim*). The mobile drawer's scrim and edge are left out of that PR because LibreChat-AI#16248 owns client/src/components/UnifiedSidebar/**, including mobile/Scrim.tsx, mobile/__tests__/Scrim.spec.tsx and UnifiedSidebar.tsx.

Work to do once LibreChat-AI#16248 merges, closing three earlier issues together:

• [upstream #16262] #16262: the mobile drawer scrim at 50% does not separate the drawer by 3:1 in any palette #105: mobile/Scrim.tsx paints bg-surface-overlay/50, which separates the drawer by 3:1 in no palette. Move it to bg-scrim (the dialogs' theme-owned 80%), which clears 3:1 in both light palettes (4.01 LibreChat, 4.09 ClickHouse). This is a deliberate default change and fixes an a11y failure.
• [upstream #16262] #16262: the mobile drawer has no 3:1 boundary against its scrim in dark mode #145: in dark mode no scrim opacity separates the near-black drawer from the page (1.17 LibreChat, 1.42 ClickHouse at 100%), so the drawer container in UnifiedSidebar.tsx needs an edge of its own with 3:1 against surface-primary-alt, and the mobile-drawer-scrim scenarios should measure dark as well.
• [upstream #16041] 16041: Scrim.spec.tsx pins Tailwind class names instead of the focus behaviour #65: replace the class-name assertions in Scrim.spec.tsx with a check of what renders (the focused scrim's computed outline or box-shadow in the mock harness).