fix: replace all remaining old brand tokens (text-brand-dark, border-brand-border, bg-white opacity)

30 references to removed CSS variables: border-brand-border → border-[#1e2130],
text-brand-text-muted → text-[#4f5666], text-brand-dark → text-white,
bg-white/[0.04] → bg-[#191c25], hover:border-white/[0.12] → hover:border-[#2a2f3d].

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/components/assistant/SuggestedFlowCard.tsx

@@ -18,7 +18,7 @@ export function SuggestedFlowCard({ flow }: SuggestedFlowCardProps) {
   return (
     <button
       onClick={handleClick}
-      className="w-full text-left card-flat p-3 rounded-xl hover:border-white/[0.12] transition-colors group"
+      className="w-full text-left card-flat p-3 rounded-xl hover:border-[#2a2f3d] transition-colors group"
     >
       <div className="flex items-start gap-2">
         <Box size={14} className="text-[#22d3ee] mt-0.5 shrink-0" />