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/tree-editor/TreeEditorLayout.tsx

@@ -18,7 +18,7 @@ export function TreeEditorLayout({ isMobile = false }: TreeEditorLayoutProps) {
       {/* Left Panel - Form Editor */}
       <div
         className={cn(
-          'flex flex-col overflow-y-auto border-border bg-background',
+          'flex flex-col overflow-y-auto border-white/[0.06]',
           isMobile ? 'h-full w-full border-b' : 'w-3/5 border-r'
         )}
       >
@@ -31,7 +31,7 @@ export function TreeEditorLayout({ isMobile = false }: TreeEditorLayoutProps) {
       {/* Right Panel - Preview */}
       <div
         className={cn(
-          'flex-1 overflow-hidden bg-muted/30',
+          'flex-1 overflow-hidden bg-white/[0.02]',
           isMobile ? 'hidden' : 'block'
         )}
       >