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/components/AuthPromptModal.tsx

@@ -0,0 +1,45 @@
+import { Link } from "@tanstack/react-router";
+import { useUIStore } from "../stores/uiStore";
+
+export function AuthPromptModal() {
+	const showAuthPrompt = useUIStore((s) => s.showAuthPrompt);
+	const closeAuthPrompt = useUIStore((s) => s.closeAuthPrompt);
+
+	if (!showAuthPrompt) return null;
+
+	return (
+		<div className="fixed inset-0 z-50 flex items-center justify-center">
+			<div
+				className="absolute inset-0 bg-black/30"
+				onClick={closeAuthPrompt}
+				onKeyDown={(e) => {
+					if (e.key === "Escape") closeAuthPrompt();
+				}}
+			/>
+			<div className="relative bg-white rounded-xl shadow-lg p-6 max-w-sm mx-4 w-full">
+				<h3 className="text-lg font-semibold text-gray-900 mb-2">
+					Join GearBox
+				</h3>
+				<p className="text-sm text-gray-600 mb-6">
+					To manage your own collection, sign in or sign up.
+				</p>
+				<div className="flex flex-col gap-3">
+					<Link
+						to="/login"
+						className="w-full text-center px-4 py-2.5 text-sm font-medium text-white bg-gray-700 hover:bg-gray-800 rounded-lg transition-colors"
+						onClick={closeAuthPrompt}
+					>
+						Sign in
+					</Link>
+					<Link
+						to="/login"
+						className="w-full text-center px-4 py-2.5 text-sm font-medium text-gray-700 bg-gray-100 hover:bg-gray-200 rounded-lg transition-colors"
+						onClick={closeAuthPrompt}
+					>
+						Create account
+					</Link>
+				</div>
+			</div>
+		</div>
+	);
+}