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/PageLoader.tsx

@@ -1,9 +1,9 @@
 export function PageLoader() {
   return (
-    <div className="flex h-screen items-center justify-center bg-background">
+    <div className="flex h-screen items-center justify-center bg-black">
       <div className="flex flex-col items-center gap-4">
-        <div className="h-12 w-12 animate-spin rounded-full border-4 border-primary border-t-transparent" />
-        <p className="text-sm text-muted-foreground">Loading...</p>
+        <div className="h-12 w-12 animate-spin rounded-full border-4 border-white/20 border-t-white" />
+        <p className="text-sm text-white/40">Loading...</p>
       </div>
     </div>
   )