[upstream #16248] 16248: a site banner covers the mobile drawer's header strip #218

Seen while verifying LibreChat-AI#16248 at a 390px phone width.

When a site banner is configured (for example the dev config's "Welcome to LibreChat!" banner), client/src/components/Banners/Banner.tsx renders sticky top-0 z-20 and paints over the top of the opened mobile drawer (client/src/components/UnifiedSidebar/UnifiedSidebar.tsx), covering the drawer's header strip: the close toggle, the panel switcher, New chat and the account menu. The drawer's own close control is unreachable by pointer until the banner is dismissed.

Neither the banner nor the drawer's stacking changes in LibreChat-AI#16248, so this already happens on canary.

Expected: the open drawer sits above the banner (or the drawer starts below it), so its header strip stays visible and operable. Add a mobile scenario with a configured banner that opens the drawer and clicks its close control.