Upgrade to Next.js 16.1.6, React 19.2.4, and update all dependencies

- Next.js 15.5 → 16.1.6 (Turbopack default, faster builds)
- React/React-DOM 19.1 → 19.2.4
- @types/react 18 → 19, @types/react-dom 18 → 19
- TypeScript 5.2 → 5.9
- CI Node.js 18 → 20 (required by Next.js 16)
- Replace removed next lint with standalone ESLint config
- Auto-updated tsconfig.json (jsx: react-jsx, dev types)

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/types/routes.d.ts";
 
 // NOTE: This file should not be edited
 // see https://nextjs.org/docs/app/api-reference/config/typescript for more information.