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/StarRating.tsx

@@ -48,14 +48,14 @@ export function StarRating({
               sizeClasses[size],
               star <= value
                 ? 'fill-yellow-400 text-yellow-400'
-                : 'fill-none text-muted-foreground',
+                : 'fill-none text-white/30',
               !readonly && 'hover:text-yellow-300'
             )}
           />
         </button>
       ))}
       {showCount && (
-        <span className="ml-1 text-sm text-muted-foreground">
+        <span className="ml-1 text-sm text-white/40">
           ({value}/5)
         </span>
       )}