Bump next from 15.5.15 to 15.5.18 in /frontend (#567)

Bumps [next](https://github.com/vercel/next.js) from 15.5.15 to 15.5.18.
- [Release notes](https://github.com/vercel/next.js/releases)
- [Changelog](https://github.com/vercel/next.js/blob/canary/release.js)
- [Commits](vercel/next.js@v15.5.15...v15.5.18)

---
updated-dependencies:
- dependency-name: next
  dependency-version: 15.5.18
  dependency-type: direct:production
...

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

frontend/package-lock.json

@@ -14,7 +14,7 @@
   },
   "dependencies": {
     "express": "4.22.1",
-    "next": "15.5.15",
+    "next": "15.5.18",
     "react": "19.0.3",
     "react-dom": "19.0.3"
   },