Upgrade to Next.js 16, enable SSR, update packages

- Upgrade all packages to latest versions
- Enable SSR (dynamic = 'force-dynamic') for all pages
- Update PostCSS config for Tailwind CSS v4 (@tailwindcss/postcss)
- Fix Edge-compatible CSRF token generation (Web Crypto API)
- Remove deprecated eslint config from next.config.ts

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

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.