Add type annotations to federation-choose-username defcomp params

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

federation/sx/auth.sx

@@ -1,7 +1,7 @@
 ;; Auth components (choose username — federation-specific)
 ;; Login and check-email components are shared: see shared/sx/templates/auth.sx
 
-(defcomp ~federation-choose-username (&key domain error csrf username check-url)
+(defcomp ~federation-choose-username (&key (domain :as string) error (csrf :as string) (username :as string) (check-url :as string))
   (div :class "py-8 max-w-md mx-auto"
     (h1 :class "text-2xl font-bold mb-2" "Choose your username")
     (p :class "text-stone-600 mb-6" "This will be your identity on the fediverse: "