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/ui/Button.tsx

@@ -8,9 +8,9 @@ const buttonVariants = cva(
     variants: {
       variant: {
         primary:
-          'bg-[#22d3ee] text-brand-dark font-semibold hover:brightness-110',
+          'bg-[#22d3ee] text-white font-semibold hover:brightness-110',
         secondary:
-          'bg-white/[0.04] border border-brand-border text-[#e2e5eb] hover:border-white/[0.12] hover:bg-brand-border',
+          'bg-[#191c25] border border-[#1e2130] text-[#e2e5eb] hover:border-[#2a2f3d] hover:bg-[#1e2130]',
         destructive:
           'bg-red-400/10 text-red-400 border border-red-400/20 hover:bg-red-400/20',
         ghost:

frontend/src/components/ui/Skeleton.tsx

@@ -47,7 +47,7 @@ export function ListSkeleton({ count = 5, className }: { count?: number; classNa
   return (
     <div className={cn('space-y-3', className)}>
       {Array.from({ length: count }).map((_, i) => (
-        <div key={i} className="flex items-center gap-3 px-4 py-3 rounded-lg bg-white/[0.02]">
+        <div key={i} className="flex items-center gap-3 px-4 py-3 rounded-lg bg-[#14161d]">
           <Skeleton className="size-8 rounded-full shrink-0" />
           <div className="flex-1 space-y-2">
             <Skeleton className="h-4 w-2/3" />