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/common/ContextMenu.tsx

@@ -87,7 +87,7 @@ export function ContextMenu({ position, items, onClose }: ContextMenuProps) {
               'flex w-full items-center gap-2 rounded-lg px-3 py-2 text-sm transition-colors',
               item.variant === 'danger'
                 ? 'text-rose-400 hover:bg-rose-500/10'
-                : 'text-[#e2e5eb] hover:bg-brand-border'
+                : 'text-[#e2e5eb] hover:bg-[#1e2130]'
             )}
           >
             {item.icon && (