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>

postcss.config.js

@@ -1,6 +1,6 @@
 module.exports = {
   plugins: {
-    tailwindcss: {},
+    '@tailwindcss/postcss': {},
     autoprefixer: {},
   },
 };