fix: use Hugo content-based 404 pages for proper multilingual support

fix: use Hugo content-based 404 pages for proper multilingual support

Created language-specific 404 pages (content/en/404.md and content/it/404.md)
instead of using JavaScript detection. Hugo now properly routes 404 errors to
the correct language context, allowing .Lang and i18n to work correctly.
Removed JS language detection hack.

Co-Authored-By: Claude Haiku 4.5 <redacted>