chore(deps): bump next from 15.5.18 to 15.5.21

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
...

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

Author: dependabot[bot]

package.json

@@ -75,7 +75,7 @@
     "livekit-client": "2.9.9",
     "lodash": "4.18.1",
     "lucide-react": "0.454.0",
-    "next": "15.5.18",
+    "next": "15.5.21",
     "next-themes": "0.4.6",
     "next-transpile-modules": "10.0.1",
     "pdfjs-dist": "5.5.207",