fix(ui): use arbitrary opacity for AuthCard dark background

Tailwind's default opacity scale only includes multiples of 5.
The `/92` modifier in `dark:bg-[#1b2331]/92` was silently ignored,
causing the light-mode card background to render in dark mode.
Changed to `/[0.92]` which uses Tailwind's arbitrary value syntax.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

packages/ui/src/components/AuthSurface.tsx

@@ -58,7 +58,7 @@ export function AuthCard({ children, className, ...props }: AuthCardProps): Reac
   return (
     <div
       className={joinClassNames(
-        "relative w-full rounded-2xl border border-[#b8c4de] bg-[#dde4f2]/90 p-6 shadow-[0_30px_70px_rgba(15,20,29,0.24)] backdrop-blur-sm sm:p-10 dark:border-[#2f3b52] dark:bg-[#1b2331]/92 dark:shadow-[0_32px_80px_rgba(0,0,0,0.52)]",
+        "relative w-full rounded-2xl border border-[#b8c4de] bg-[#dde4f2]/90 p-6 shadow-[0_30px_70px_rgba(15,20,29,0.24)] backdrop-blur-sm sm:p-10 dark:border-[#2f3b52] dark:bg-[#1b2331]/[0.92] dark:shadow-[0_32px_80px_rgba(0,0,0,0.52)]",
         className
       )}
       {...props}