refactor(deps): pin dependency @tailwindcss/postcss to 4.0.15

Author: renovate[bot]

package.json

@@ -7,7 +7,7 @@
   "private": true,
   "dependencies": {
     "@fortawesome/fontawesome-free": "6.7.2",
-    "@tailwindcss/postcss": "^4.0.15",
+    "@tailwindcss/postcss": "4.0.15",
     "@tailwindcss/typography": "0.5.16",
     "alpinejs": "3.14.8",
     "hugo-bin": "0.142.0",