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/stores/uiStore.ts

@@ -79,6 +79,11 @@ interface UIState {
 	// Session thread tracking
 	catalogSessionThreadId: number | null;
 	setCatalogSessionThreadId: (id: number | null) => void;
+
+	// Auth prompt modal
+	showAuthPrompt: boolean;
+	openAuthPrompt: () => void;
+	closeAuthPrompt: () => void;
 }
 
 export const useUIStore = create<UIState>((set) => ({
@@ -184,4 +189,9 @@ export const useUIStore = create<UIState>((set) => ({
 	// Session thread tracking
 	catalogSessionThreadId: null,
 	setCatalogSessionThreadId: (id) => set({ catalogSessionThreadId: id }),
+
+	// Auth prompt modal
+	showAuthPrompt: false,
+	openAuthPrompt: () => set({ showAuthPrompt: true }),
+	closeAuthPrompt: () => set({ showAuthPrompt: false }),
 }));