chore(deps): update nextjs monorepo to v15.5.12

Author: renovate[bot]

package.json

@@ -46,7 +46,7 @@
     "@mui/lab": "7.0.1-beta.20",
     "@mui/material": "7.3.6",
     "@mui/material-nextjs": "7.3.6",
-    "@next/mdx": "15.5.9",
+    "@next/mdx": "15.5.12",
     "@rjsf/core": "6.1.2",
     "@rjsf/mui": "6.1.2",
     "@rjsf/utils": "6.1.2",
@@ -82,7 +82,7 @@
     "lodash-es": "4.17.22",
     "material-ui-popup-state": "5.3.6",
     "nanoid": "5.1.6",
-    "next": "15.5.9",
+    "next": "15.5.12",
     "next-http-proxy-middleware": "1.2.7",
     "nextjs-routes": "2.2.5",
     "node-fetch": "3.3.2",