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/common/UpgradePrompt.tsx

@@ -11,17 +11,17 @@ export function UpgradePrompt({ feature, className }: UpgradePromptProps) {
 
   return (
     <div className={cn(
-      'rounded-lg border border-yellow-500/30 bg-yellow-500/10 p-4',
+      'glass-card rounded-2xl border border-white/[0.06] p-4',
       className
     )}>
-      <h3 className="font-semibold text-foreground">Plan Limit Reached</h3>
-      <p className="mt-1 text-sm text-muted-foreground">
+      <h3 className="font-semibold text-white">Plan Limit Reached</h3>
+      <p className="mt-1 text-sm text-white/70">
         Your {plan} plan doesn't allow you to {feature}. Upgrade your plan to continue.
       </p>
       <button
         className={cn(
-          'mt-3 rounded-md bg-primary px-4 py-2 text-sm font-medium text-primary-foreground',
-          'hover:bg-primary/90'
+          'mt-3 rounded-xl bg-white px-4 py-2 text-sm font-medium text-black',
+          'hover:bg-white/90'
         )}
         onClick={() => window.location.href = '/account'}
       >