chore: update dependencies and configuration for Tailwind CSS

- Added "@tailwindcss/postcss" to package.json dependencies.
- Updated "tailwindcss" version to "^4.3.0" in package.json.
- Modified postcss.config.mjs to use "@tailwindcss/postcss" instead of "tailwindcss".
- Changed darkMode configuration in tailwind.config.ts to a string format.

next-env.d.ts

@@ -1,6 +1,6 @@
 /// <reference types="next" />
 /// <reference types="next/image-types/global" />
-import "./.next/dev/types/routes.d.ts";
+import "./.next/types/routes.d.ts";
 
 // NOTE: This file should not be edited
 // see https://nextjs.org/docs/app/api-reference/config/typescript for more information.