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/library/ImportFlowModal.tsx

@@ -122,7 +122,7 @@ export function ImportFlowModal({ onClose }: ImportFlowModalProps) {
                   'flex flex-col items-center justify-center rounded-lg border-2 border-dashed px-4 py-8 text-center transition-colors cursor-pointer',
                   isDragging
                     ? 'border-primary/50 bg-primary/5'
-                    : 'border-[#1e2130] hover:border-white/[0.12]'
+                    : 'border-[#1e2130] hover:border-[#2a2f3d]'
                 )}
                 onClick={() => fileInputRef.current?.click()}
                 onDragOver={(e) => { e.preventDefault(); setIsDragging(true) }}