Site owners can now set a default language for the consent banner,
overriding browser auto-detection. When null the banner uses
navigator.language / data-locale as before (existing behaviour).
Changes:
- DB: add default_language column to site_configs (nullable, String(10))
- API model: SiteConfig.default_language field
- API schema: SiteConfigCreate/Update/Response schemas
- Config resolver: pass default_language through to public config
- Banner types: SiteConfig.default_language field
- Banner logic: use config.default_language ?? detectLocale()
- Admin UI: SiteConfigTab dropdown with auto-detect option
- Admin UI types: SiteConfig.default_language added
