fix(deps): update nextjs monorepo to v15.5.6

Author: renovate[bot]

package.json

@@ -41,7 +41,7 @@
     "@vercel/speed-insights": "^1.1.0",
     "classnames": "^2.5.1",
     "jotai": "^2.9.0",
-    "next": "15.2.4",
+    "next": "15.5.6",
     "react": "^19.0.0",
     "react-dom": "^19.0.0"
   },