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/BrandWordmark.tsx

@@ -6,20 +6,19 @@ interface BrandWordmarkProps {
 }
 
 /**
- * ResolutionFlow wordmark with gradient "Flow" text.
+ * ResolutionFlow wordmark — clean white text.
  * sm for header/navbar, lg for login/register pages.
  */
 export function BrandWordmark({ size = 'sm', className }: BrandWordmarkProps) {
   return (
     <span
       className={cn(
-        'font-heading font-bold',
-        size === 'sm' ? 'text-xl' : 'text-3xl tracking-tight',
+        'font-semibold tracking-tight text-white',
+        size === 'sm' ? 'text-xl' : 'text-3xl',
         className
       )}
     >
-      <span className="text-foreground">Resolution</span>
-      <span className="text-gradient-brand">Flow</span>
+      ResolutionFlow
     </span>
   )
 }