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/AdminSidebar.tsx

@@ -39,7 +39,7 @@ export function AdminSidebar({ className, onNavigate }: AdminSidebarProps) {
   return (
     <aside className={cn('flex h-full flex-col', className)}>
       <div className="p-4">
-        <h2 className="font-heading text-lg font-bold text-foreground">Admin Panel</h2>
+        <h2 className="text-lg font-bold text-white">Admin Panel</h2>
       </div>
       <nav className="flex-1 space-y-1 px-3">
         {navItems.map((item) => (
@@ -50,8 +50,8 @@ export function AdminSidebar({ className, onNavigate }: AdminSidebarProps) {
             className={cn(
               'flex items-center gap-3 rounded-md px-3 py-2 text-sm font-medium transition-colors',
               isActive(item.path, item.end)
-                ? 'bg-accent text-accent-foreground'
-                : 'text-muted-foreground hover:bg-accent hover:text-accent-foreground'
+                ? 'bg-white/10 text-white'
+                : 'text-white/50 hover:bg-white/[0.06] hover:text-white'
             )}
           >
             <item.icon className="h-4 w-4" />
@@ -59,13 +59,13 @@ export function AdminSidebar({ className, onNavigate }: AdminSidebarProps) {
           </Link>
         ))}
       </nav>
-      <div className="border-t border-border p-3">
+      <div className="border-t border-white/[0.06] p-3">
         <Link
           to="/trees"
           onClick={onNavigate}
           className={cn(
             'flex items-center gap-3 rounded-md px-3 py-2 text-sm font-medium',
-            'text-muted-foreground hover:bg-accent hover:text-accent-foreground'
+            'text-white/50 hover:bg-white/[0.06] hover:text-white'
           )}
         >
           <ArrowLeft className="h-4 w-4" />