Bump next from 15.5.18 to 15.5.21 in /web (#557)

Bumps [next](https://github.com/vercel/next.js) from 15.5.18 to 15.5.21.
- [Release notes](https://github.com/vercel/next.js/releases)
- [Commits](vercel/next.js@v15.5.18...v15.5.21)

---
updated-dependencies:
- dependency-name: next
  dependency-version: 15.5.21
  dependency-type: direct:production
...

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

Author: dependabot[bot]

web/package.json

@@ -32,7 +32,7 @@
     "clsx": "^2.1.1",
     "lucide-react": "^1.16.0",
     "msw": "^2.14.6",
-    "next": "15.5.18",
+    "next": "15.5.21",
     "next-auth": "5.0.0-beta.32",
     "next-themes": "^0.4.6",
     "react": "19.2.4",