Add missing return after redirect in /login handler

Without the return, execution fell through to render the login page
into the same response after writing the redirect header.

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

cmd/web/routes.go

@@ -39,6 +39,7 @@ func routes(r *chi.Mux) {
 	r.Get("/login", func(w http.ResponseWriter, r *http.Request) {
 		if ValidateJWTFromCookie(r, appKey) {
 			http.Redirect(w, r, "/setup", http.StatusSeeOther)
+			return
 		}
 		comp := templates.Login()
 		comp.Render(context.Background(), w)