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/library/SortDropdown.tsx

@@ -21,7 +21,7 @@ const sortOptions: { value: SortBy; label: string }[] = [
 export function SortDropdown({ value, onChange, className }: SortDropdownProps) {
   return (
     <div className={cn('relative inline-flex items-center', className)}>
-      <span className="mr-2 flex items-center gap-1.5 text-sm text-muted-foreground">
+      <span className="mr-2 flex items-center gap-1.5 text-sm text-white/40">
         <ArrowUpDown className="h-4 w-4" />
         <span className="hidden sm:inline">Sort:</span>
       </span>
@@ -29,8 +29,8 @@ export function SortDropdown({ value, onChange, className }: SortDropdownProps)
         value={value}
         onChange={(e) => onChange(e.target.value as SortBy)}
         className={cn(
-          'rounded-md border border-input bg-background px-3 py-1.5 text-sm',
-          'text-foreground focus:border-primary focus:outline-none focus:ring-1 focus:ring-primary'
+          'rounded-md border border-white/10 bg-black/50 px-3 py-1.5 text-sm',
+          'text-white focus:border-white/30 focus:outline-none focus:ring-1 focus:ring-white/20'
         )}
       >
         {sortOptions.map((option) => (