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/PageHeader.tsx

@@ -12,7 +12,7 @@ export function PageHeader({ title, description, action, className }: PageHeader
   return (
     <div className={cn('flex items-start justify-between gap-4', className)}>
       <div>
-        <h1 className="font-heading text-2xl font-bold text-foreground">{title}</h1>
+        <h1 className="text-2xl font-bold text-foreground">{title}</h1>
         {description && (
           <p className="mt-1 text-sm text-muted-foreground">{description}</p>
         )}