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/procedural/FallbackSteps.tsx

@@ -64,7 +64,7 @@ export function FallbackSteps({
                   key={fbStep.id}
                   className={cn(
                     'rounded-lg border p-3 transition-colors',
-                    'bg-white/[0.02] border-[#1e2130]/50',
+                    'bg-[#14161d] border-[#1e2130]/50',
                     isCompleted && 'border-emerald-500/30 bg-emerald-500/5'
                   )}
                 >