Upgrade to Next.js 16, TypeScript 5.9, @types/node 24

Bumps all dependencies to latest compatible versions:
- next 15 → 16.2.1
- typescript 5 → 5.9.3
- @types/node 22 → 24.12.0
- eslint-config-next 15 → 16.2.1
- eslint stays at 9.x (plugin ecosystem lacks ESLint 10 support)

Includes tsconfig/next-env changes auto-applied by Next.js 16.

Author: planetf1

next-env.d.ts

@@ -1,6 +1,6 @@
 /// <reference types="next" />
 /// <reference types="next/image-types/global" />
-/// <reference path="./.next/types/routes.d.ts" />
+import "./.next/dev/types/routes.d.ts";
 
 // NOTE: This file should not be edited
 // see https://nextjs.org/docs/app/api-reference/config/typescript for more information.