chore(deps): update dependency tailwindcss to v4

Author: renovate[bot]

apps/penpal/frontend/package.json

@@ -42,7 +42,7 @@
     "eslint": "^10.0.0",
     "jsdom": "^25.0.1",
     "postcss": "^8.4.0",
-    "tailwindcss": "^3.4.0",
+    "tailwindcss": "^4.0.0",
     "typescript": "~5.9.0",
     "unist-util-visit": "^5.1.0",
     "vite": "^6.0.0",