Make self-service signup opt-in via SIGNUP_ENABLED

/users/signup answers 403 unless SIGNUP_ENABLED is set. A public
GET /utils/config/ tells the login page, which hides the sign-up link,
and the signup page explains that an administrator creates accounts.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_013GAeg4Jo6xjZzjwy9QDWMQ

frontend/src/hooks/useAuth.ts

@@ -7,6 +7,7 @@ import {
   type UserPublic,
   type UserRegister,
   UsersService,
+  UtilsService,
 } from "@/client"
 import { liveStore } from "@/components/Flow/liveStore"
 import { isPortal } from "@/lib/portal"
@@ -32,6 +33,14 @@ export function safeRedirect(target: string | undefined): string {
   return target
 }
 
+/** Whether this instance lets a visitor create an account; unknown while loading. */
+export const useSignupEnabled = (): boolean | undefined =>
+  useQuery({
+    queryKey: ["config"],
+    queryFn: UtilsService.readConfig,
+    staleTime: Number.POSITIVE_INFINITY,
+  }).data?.signup_enabled
+
 const useAuth = () => {
   const navigate = useNavigate()
   const queryClient = useQueryClient()