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>

package.json

@@ -70,7 +70,6 @@
     "motion": "^11.15.0",
     "next": "^15.1.0",
     "next-auth": "^5.0.0-beta.25",
-    "next-intl": "^4.8.2",
     "next-themes": "^0.4.6",
     "nodemailer": "^7.0.7",
     "openai": "^6.16.0",