feat(24-02): add auth prompt state, modal, usePublicSetup hook, guard onboarding

- Extend uiStore with showAuthPrompt/openAuthPrompt/closeAuthPrompt state
- Create AuthPromptModal component with sign in/sign up CTAs pointing to /login
- Add usePublicSetup hook to useSetups for anonymous setup viewing via public API
- Rework useOnboardingComplete to accept enabled param (guards auth-gated call)

src/client/hooks/useSetups.ts

@@ -64,6 +64,16 @@ export function useSetup(setupId: number | null) {
 	});
 }
 
+export function usePublicSetup(setupId: number | null) {
+	return useQuery({
+		queryKey: ["setups", setupId, "public"],
+		queryFn: () => apiGet<SetupWithItems>(`/api/setups/${setupId}/public`),
+		enabled: setupId != null,
+		retry: (count, error) =>
+			error instanceof ApiError && error.status === 404 ? false : count < 3,
+	});
+}
+
 export function useCreateSetup() {
 	const queryClient = useQueryClient();
 	return useMutation({