feat: add default_language to site config for banner i18n

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

apps/banner/src/banner.ts

@@ -220,7 +220,8 @@ async function init(): Promise<void> {
   }
 
   // Load translations
-  const locale = detectLocale();
+  // Use site-configured default_language if set, otherwise auto-detect
+  const locale = config.default_language ?? detectLocale();
   const t = await loadTranslations(cdnBase, locale);
 
   // Capture a closure that re-opens the banner with current consent

apps/banner/src/types.ts

@@ -80,6 +80,7 @@ export interface SiteConfig {
   gcm_default: Record<string, 'granted' | 'denied'> | null;
   shopify_privacy_enabled: boolean;
   banner_config: BannerConfig | null;
+  default_language: string | null;
   privacy_policy_url: string | null;
   terms_url: string | null;
   consent_expiry_days: number;