fix: update Next.js to 15.5.12 to patch CVE-2025-66478

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: bernatfp

package.json

@@ -16,7 +16,7 @@
     "prepare": "husky"
   },
   "dependencies": {
-    "next": "^15.0.2",
+    "next": "^15.5.12",
     "nextra": "^4.0.0",
     "nextra-theme-docs": "^4.0.0",
     "react": "^19.1.0",