Update nextjs monorepo to v15

Author: renovate[bot]

webui/package.json

@@ -36,7 +36,7 @@
     "axios": "^1.8.1",
     "clsx": "^2.1.1",
     "dayjs": "^1.11.13",
-    "next": "14.2.24",
+    "next": "15.2.1",
     "react": "^18.3.1",
     "react-digraph": "^9.1.2",
     "react-dom": "^18.3.1",
@@ -49,7 +49,7 @@
     "@types/react": "^18.3.18",
     "@types/react-dom": "^18.3.5",
     "eslint": "^8.57.1",
-    "eslint-config-next": "14.2.24",
+    "eslint-config-next": "15.2.1",
     "postcss": "^8.5.3",
     "postcss-preset-mantine": "^1.17.0",
     "postcss-simple-vars": "^7.0.1",