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/ActionMenu.tsx

@@ -53,8 +53,8 @@ export function ActionMenu({ items }: ActionMenuProps) {
         ref={buttonRef}
         onClick={() => setOpen(!open)}
         className={cn(
-          'rounded-md p-1.5 text-muted-foreground transition-colors',
-          'hover:bg-accent hover:text-accent-foreground'
+          'rounded-md p-1.5 text-white/50 transition-colors',
+          'hover:bg-white/10 hover:text-white'
         )}
       >
         <MoreHorizontal className="h-4 w-4" />
@@ -63,8 +63,8 @@ export function ActionMenu({ items }: ActionMenuProps) {
         <div
           ref={menuRef}
           className={cn(
-            'fixed z-50 min-w-[160px] rounded-md border border-border',
-            'bg-card py-1 shadow-lg animate-scale-in'
+            'fixed z-50 min-w-[160px] rounded-md border border-white/10',
+            'bg-black py-1 shadow-lg animate-scale-in'
           )}
           style={{
             top: `${menuPosition.top}px`,
@@ -80,8 +80,8 @@ export function ActionMenu({ items }: ActionMenuProps) {
                 'flex w-full items-center gap-2 px-3 py-2 text-sm transition-colors',
                 'disabled:opacity-50 disabled:pointer-events-none',
                 item.destructive
-                  ? 'text-destructive hover:bg-destructive/10'
-                  : 'text-foreground hover:bg-accent'
+                  ? 'text-red-400 hover:bg-red-400/10'
+                  : 'text-white/70 hover:bg-white/[0.06]'
               )}
             >
               {item.icon}