chore: Bump next from 15.3.4 to 15.5.6

Bumps [next](https://github.com/vercel/next.js) from 15.3.4 to 15.5.6.
- [Release notes](https://github.com/vercel/next.js/releases)
- [Changelog](https://github.com/vercel/next.js/blob/canary/release.js)
- [Commits](vercel/next.js@v15.3.4...v15.5.6)

---
updated-dependencies:
- dependency-name: next
  dependency-version: 15.5.6
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

frontend/package.json

@@ -38,7 +38,7 @@
     "@headlessui/react": "^2.2.4",
     "@sanity/image-url": "^1.1.0",
     "gsap": "^3.13.0",
-    "next": "15.3.4",
+    "next": "16.0.0",
     "next-sanity": "^2.1.0",
     "next-view-transitions": "^0.3.4",
     "react": "^19.0.0",