Extract detectLocale() from hooks.server.ts into src/lib/server/locale.ts
so it can be tested in isolation. Add 7 unit tests covering:
- German, English, Spanish browser preferences
- Fallback when primary language is unsupported
- Quality value (q=) ordering
- Fully unsupported language → null
- Empty Accept-Language header → null
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
