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.

tailwind.config.ts

@@ -2,7 +2,7 @@ import type { Config } from "tailwindcss";
 import tailwindcssAnimate from "tailwindcss-animate";
 
 const config: Config = {
-  darkMode: ["class"],
+  darkMode: "class",
   content: [
     "./app/**/*.{js,ts,jsx,tsx,mdx}",
     "./components/**/*.{js,ts,jsx,tsx,mdx}",