Remove next-intl localization infrastructure

Strips out the i18n layer (next-intl, message files, language switcher,
provider) to reduce complexity. Nav components now use plain English strings.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

next.config.ts

@@ -1,5 +1,4 @@
 import type { NextConfig } from 'next'
-import createNextIntlPlugin from 'next-intl/plugin'
 
 const nextConfig: NextConfig = {
   output: 'standalone',
@@ -18,6 +17,4 @@ const nextConfig: NextConfig = {
   },
 }
 
-const withNextIntl = createNextIntlPlugin('./src/i18n/request.ts')
-
-export default withNextIntl(nextConfig)
+export default nextConfig