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/admin/Pagination.tsx

@@ -36,20 +36,20 @@ export function Pagination({ page, totalPages, total, pageSize, onPageChange }:
 
   return (
     <div className="flex items-center justify-between gap-4 pt-4">
-      <span className="text-sm text-muted-foreground">
+      <span className="text-sm text-white/40">
         Showing {start}-{end} of {total}
       </span>
       <div className="flex items-center gap-1">
         <button
           onClick={() => onPageChange(page - 1)}
           disabled={page <= 1}
-          className={cn(btnBase, 'px-2 hover:bg-accent')}
+          className={cn(btnBase, 'px-2 text-white/50 hover:bg-white/[0.06] hover:text-white')}
         >
           <ChevronLeft className="h-4 w-4" />
         </button>
         {getPageNumbers().map((p, i) =>
           p === 'ellipsis' ? (
-            <span key={`e${i}`} className="px-1 text-muted-foreground">...</span>
+            <span key={`e${i}`} className="px-1 text-white/40">...</span>
           ) : (
             <button
               key={p}
@@ -58,8 +58,8 @@ export function Pagination({ page, totalPages, total, pageSize, onPageChange }:
                 btnBase,
                 'px-2',
                 p === page
-                  ? 'bg-primary text-primary-foreground'
-                  : 'hover:bg-accent text-muted-foreground'
+                  ? 'bg-white text-black'
+                  : 'text-white/50 hover:bg-white/[0.06] hover:text-white'
               )}
             >
               {p}
@@ -69,7 +69,7 @@ export function Pagination({ page, totalPages, total, pageSize, onPageChange }:
         <button
           onClick={() => onPageChange(page + 1)}
           disabled={page >= totalPages}
-          className={cn(btnBase, 'px-2 hover:bg-accent')}
+          className={cn(btnBase, 'px-2 text-white/50 hover:bg-white/[0.06] hover:text-white')}
         >
           <ChevronRight className="h-4 w-4" />
         </button>