Function negotiate_locale 

pub fn negotiate_locale(
    preferred: &[String],
    available: &[String],
    default_locale: &str,
) -> String

Given a list of preferred locales (from Accept-Language) and a list of available locales (directories on disk), returns the best match.

Matching rules:

1. Exact match (e.g., “fr-CH” matches “fr-CH”)
2. Prefix match (e.g., “fr-CH” matches “fr”)
3. Default locale fallback