refactor: redesign planos interface with design system patterns

- Create CreatePlanModal component with Headless UI Dialog
- Implement dark mode support throughout plans UI
- Update plans/page.tsx with professional card layout
- Update plans/[id]/page.tsx with consistent styling
- Add proper spacing, typography, and color consistency
- Implement smooth animations and transitions
- Add success/error message feedback
- Improve form UX with better input styling

front-end-dash.aggios.app/app/superadmin/layout.tsx

@@ -7,11 +7,13 @@ import {
     LinkIcon,
     DocumentTextIcon,
     Cog6ToothIcon,
+    SparklesIcon,
 } from '@heroicons/react/24/outline';
 
 const SUPERADMIN_MENU_ITEMS = [
     { id: 'dashboard', label: 'Dashboard', href: '/superadmin', icon: HomeIcon },
     { id: 'agencies', label: 'Agências', href: '/superadmin/agencies', icon: BuildingOfficeIcon },
+    { id: 'plans', label: 'Planos', href: '/superadmin/plans', icon: SparklesIcon },
     { id: 'templates', label: 'Templates', href: '/superadmin/signup-templates', icon: LinkIcon },
     { id: 'agency-templates', label: 'Templates Agência', href: '/superadmin/agency-templates', icon: DocumentTextIcon },
     { id: 'settings', label: 'Configurações', href: '/superadmin/settings', icon: Cog6ToothIcon },