fix(frontend): update Next.js to 14.2.15

Fix security vulnerability in Next.js 14.0.4.
See: https://nextjs.org/blog/security-update-2025-12-11

Other npm deprecation warnings are from transitive dependencies
(@solana/wallet-adapter packages) and don't affect functionality.

frontend/package.json

@@ -22,7 +22,7 @@
     "class-variance-authority": "^0.7.0",
     "clsx": "^2.0.0",
     "lucide-react": "^0.294.0",
-    "next": "14.0.4",
+    "next": "14.2.15",
     "react": "^18.2.0",
     "react-dom": "^18.2.0",
     "recharts": "^2.10.3",