chore(dependencies): update Tailwind CSS and related packages for improved styling

- Added @tailwindcss/postcss as a new dependency.
- Updated tailwindcss to version 4.1.11 for enhanced features and performance.
- Upgraded prettier-plugin-tailwindcss to version 0.6.13 for better formatting support.
- Adjusted postcss.config.js to use the new Tailwind CSS plugin structure.
- Removed the obsolete tailwind.config.js file as it is no longer needed.

worklenz-frontend/src/pages/TaskManagementDemo.tsx

@@ -20,7 +20,7 @@ const TaskManagementDemo: React.FC = () => {
 
   return (
     <Layout className="min-h-screen bg-gray-50">
-      <Header className="bg-white shadow-sm">
+      <Header className="bg-white shadow-xs">
         <div className="max-w-7xl mx-auto px-4">
           <Title level={2} className="mb-0 text-gray-800">
             Enhanced Task Management System