Expose showBranding as read-only GraphQL field and reflect it in theme preview

Uses the Loader + Page + Fragment pattern so the snippet page fetches
its own data instead of relying on Outlet context.

Signed-off-by: Émile Ré <emile@getprobo.com>

pkg/server/api/console/v1/graphql/cookie_banner.graphql

@@ -84,6 +84,7 @@ type CookieBanner implements Node {
     privacyPolicyUrl: String!
     consentExpiryDays: Int!
     consentMode: CookieConsentMode!
+    showBranding: Boolean!
 
     organization: Organization @goField(forceResolver: true)