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

This reverts commit f69626d564.

demo/nextjs/postcss.config.mjs

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