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/pages/VerifyEmailPage.tsx

@@ -55,8 +55,8 @@ export function VerifyEmailPage() {
             <Link
               to="/"
               className={cn(
-                'mt-6 inline-flex items-center rounded-[10px] bg-[rgba(255,255,255,0.04)] border border-brand-border px-6 py-2 text-sm font-medium text-foreground',
-                'hover:border-[rgba(255,255,255,0.12)]'
+                'mt-6 inline-flex items-center rounded-[10px] bg-white/[0.04] border border-brand-border px-6 py-2 text-sm font-medium text-foreground',
+                'hover:border-white/[0.12]'
               )}
             >
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