fix: restore locale redirection using Next.js middleware

[Biki-dev]

Summary

Restored locale-based redirection by reintroducing the logic as a proper Next.js middleware.ts.

Changes

• Added middleware.ts to handle locale detection and redirection
• Added cookie support to persist user locale

solves #458

[broBinChen]

In Next.js version 16, the middleware file convention has been deprecated and should now be replaced with proxy.ts. For more details, please refer to: https://nextjs.org/docs/messages/middleware-to-proxy

[DayuanJiang]

@broBinChen Yes, I accidentally deleted the proxy.ts file. Putting it back should fix it.

[Biki-dev]

@broBinChen Thanks, yes — that was my mistake. I used the old middleware name.