deps: update next.js to 15.5.9

Author: akshay0611

package-lock.json

@@ -104,7 +104,7 @@
     "lucide-react": "^0.511.0",
     "marked": "^17.0.0",
     "motion": "^12.23.12",
-    "next": "15.5.7",
+    "next": "15.5.9",
     "next-themes": "^0.4.6",
     "node-fetch": "^3.3.2",
     "pdf-lib": "^1.17.1",