Reapply "chore: bump tailwindcss to v4 (#4681)"

This reverts commit d1fe10962079c82920ea078a9847f65a59907b81.

demo/nextjs/package.json

@@ -90,6 +90,7 @@
     "zod": "^4.1.5"
   },
   "devDependencies": {
+    "@tailwindcss/postcss": "^4.1.13",
     "@types/canvas-confetti": "^1.9.0",
     "@types/react": "^19.1.12",
     "@types/react-dom": "^19.1.9",

demo/nextjs/postcss.config.mjs

@@ -1,8 +1,5 @@
-/** @type {import('postcss-load-config').Config} */
-const config = {
+export default {
 	plugins: {
-		tailwindcss: {},
+		"@tailwindcss/postcss": {},
 	},
 };
-
-export default config;