feat: implement monochrome design system across entire frontend

Migrate all 84 frontend files from the old themed/colored design to a
monochrome glass-morphism design system. Pure black backgrounds, white
text with opacity levels, glass-card components with backdrop-blur, and
functional color reserved for status indicators only.

Foundation: remap CSS variables to monochrome, simplify Tailwind config,
remove theme toggle, convert brand logo/wordmark to white. Pages: all
14 pages updated. Components: all common, library, session, step-library,
tree-editor, tree-preview, admin, and subscription components converted.

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

frontend/src/components/admin/EmptyState.tsx

@@ -12,10 +12,10 @@ interface EmptyStateProps {
 export function EmptyState({ icon, title, description, action, className }: EmptyStateProps) {
   return (
     <div className={cn('flex flex-col items-center justify-center py-12 text-center', className)}>
-      {icon && <div className="mb-4 text-muted-foreground">{icon}</div>}
-      <h3 className="text-lg font-semibold text-foreground">{title}</h3>
+      {icon && <div className="mb-4 text-white/50">{icon}</div>}
+      <h3 className="text-lg font-semibold text-white">{title}</h3>
       {description && (
-        <p className="mt-1 max-w-sm text-sm text-muted-foreground">{description}</p>
+        <p className="mt-1 max-w-sm text-sm text-white/40">{description}</p>
       )}
       {action && <div className="mt-4">{action}</div>}
     </div>