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/subscription/CheckoutButton.tsx

@@ -13,7 +13,7 @@ export function CheckoutButton({ plan, className }: CheckoutButtonProps) {
       disabled
       title="Billing coming soon"
       className={cn(
-        'rounded-md bg-primary px-4 py-2 text-sm font-medium text-primary-foreground',
+        'rounded-md bg-white px-4 py-2 text-sm font-medium text-black',
         'disabled:opacity-50 disabled:cursor-not-allowed',
         className
       )}