Bump next from 15.5.9 to 15.5.15 in /frontend

Bumps [next](https://github.com/vercel/next.js) from 15.5.9 to 15.5.15.
- [Release notes](https://github.com/vercel/next.js/releases)
- [Changelog](https://github.com/vercel/next.js/blob/canary/release.js)
- [Commits](vercel/next.js@v15.5.9...v15.5.15)

---
updated-dependencies:
- dependency-name: next
  dependency-version: 15.5.15
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

frontend/package-lock.json

@@ -30,7 +30,7 @@
     "@mdx-js/loader": "^3.1.1",
     "@next/mdx": "^15.0.0",
     "bootstrap": "^5.3.3",
-    "next": "^15.0.0",
+    "next": "^15.5.15",
     "next-export-optimize-images": "^4.7.0",
     "react": "^19.0.0",
     "react-bootstrap": "^2.10.10",