refactor: replace hardcoded rgba/hex colors with Tailwind tokens

- rgba(255,255,255,0.xx) → bg-white/[0.xx], border-white/[0.xx]
- rgba(6,182,212,0.3) → border-primary/30 (focus states)
- #0a0a0a → bg-background
- Inline style hex colors → var(--color-primary), var(--color-brand-gradient-to)
- 28 files updated, zero hardcoded rgba() patterns remaining

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

frontend/src/components/ui/Skeleton.tsx

@@ -45,7 +45,7 @@ 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-[rgba(255,255,255,0.02)]">
+        <div key={i} className="flex items-center gap-3 px-4 py-3 rounded-lg bg-white/[0.02]">
           <Skeleton className="size-8 rounded-full shrink-0" />
           <div className="flex-1 space-y-2">
             <Skeleton className="h-4 w-2/3" />