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/admin-ui/src/components/SiteConfigTab.tsx

@@ -115,6 +115,7 @@ export default function SiteConfigTab({ siteId, config }: Props) {
   const [consentExpiry, setConsentExpiry] = useState(config?.consent_expiry_days ?? 365);
   const [privacyUrl, setPrivacyUrl] = useState(config?.privacy_policy_url ?? '');
   const [termsUrl, setTermsUrl] = useState(config?.terms_url ?? '');
+  const [defaultLanguage, setDefaultLanguage] = useState(config?.default_language ?? '');
 
   // GPP state
   const [gppEnabled, setGppEnabled] = useState(config?.gpp_enabled ?? true);
@@ -167,6 +168,7 @@ export default function SiteConfigTab({ siteId, config }: Props) {
       consent_expiry_days: consentExpiry,
       privacy_policy_url: privacyUrl || null,
       terms_url: termsUrl || null,
+      default_language: defaultLanguage || null,
       gpp_enabled: gppEnabled,
       gpp_supported_apis: gppEnabled ? gppSupportedApis : null,
       gpc_enabled: gpcEnabled,
@@ -287,6 +289,38 @@ export default function SiteConfigTab({ siteId, config }: Props) {
               <code className="rounded bg-surface px-1">{'[Privacy Policy]({{privacy_policy}})'}</code>
             </p>
           </div>
+
+          <div>
+            <div className="flex items-center">
+              <FormField label="Default banner language">
+                <Select
+                  value={defaultLanguage}
+                  onChange={(e) => setDefaultLanguage(e.target.value)}
+                >
+                  <option value="">Auto-detect (browser language)</option>
+                  <option value="en">English</option>
+                  <option value="th">Thai</option>
+                  <option value="fr">French</option>
+                  <option value="de">German</option>
+                  <option value="es">Spanish</option>
+                  <option value="it">Italian</option>
+                  <option value="nl">Dutch</option>
+                  <option value="pt">Portuguese</option>
+                  <option value="pl">Polish</option>
+                  <option value="ja">Japanese</option>
+                  <option value="ko">Korean</option>
+                  <option value="zh">Chinese</option>
+                  <option value="ar">Arabic</option>
+                </Select>
+              </FormField>
+              <SourceBadge source={getSource('default_language')} field="default language" />
+              <ResetButton field="default_language" inheritance={inheritance} onReset={() => { setDefaultLanguage(''); markReset('default_language'); }} />
+            </div>
+            <p className="mt-1 text-xs text-text-secondary">
+              Overrides browser language detection. Set to &ldquo;Auto-detect&rdquo; to let the
+              banner use the visitor&rsquo;s browser or device language.
+            </p>
+          </div>
         </div>
       </Card>
 

apps/admin-ui/src/types/api.ts

@@ -125,6 +125,7 @@ export interface SiteConfig {
   banner_config: BannerConfig | null;
   privacy_policy_url: string | null;
   terms_url: string | null;
+  default_language: string | null;
   consent_expiry_days: number;
   scan_enabled: boolean;
   scan_frequency_hours: number;