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/common/ConfirmDialog.tsx

@@ -34,8 +34,8 @@ export function ConfirmDialog({
             onClick={onClose}
             disabled={isLoading}
             className={cn(
-              'rounded-md border border-border px-4 py-2 text-sm font-medium',
-              'text-card-foreground hover:bg-accent',
+              'rounded-xl border border-white/10 px-4 py-2 text-sm font-medium',
+              'text-white/60 hover:bg-white/10 hover:text-white',
               'disabled:opacity-50 disabled:cursor-not-allowed'
             )}
           >
@@ -45,11 +45,11 @@ export function ConfirmDialog({
             onClick={onConfirm}
             disabled={isLoading}
             className={cn(
-              'rounded-md px-4 py-2 text-sm font-medium text-white',
+              'rounded-xl px-4 py-2 text-sm font-medium',
               'disabled:opacity-50 disabled:cursor-not-allowed',
               confirmVariant === 'destructive'
-                ? 'bg-destructive hover:bg-destructive/90'
-                : 'bg-primary hover:bg-primary/90'
+                ? 'bg-red-400/10 text-red-400 hover:bg-red-400/20 border border-red-400/20'
+                : 'bg-white text-black hover:bg-white/90'
             )}
           >
             {isLoading ? 'Processing...' : confirmLabel}
@@ -57,7 +57,7 @@ export function ConfirmDialog({
         </div>
       }
     >
-      <p className="text-sm text-muted-foreground">{message}</p>
+      <p className="text-sm text-white/70">{message}</p>
     </Modal>
   )
 }