Implement user state management with Zustand and session persistence #23

📋 Description

Create a centralized user/auth store with persistent session support and explicit auth state transitions.

✅ Acceptance Criteria

• Zustand store includes user profile, auth status, role/plan, and token metadata.
• Session persistence works across page reloads.
• Explicit states exist for unauthenticated, authenticating, authenticated, and error.
• Store reset utility exists for logout/session invalidation.

🔗 Prerequisite Issues

DoD

• App restores valid session on refresh without flicker loops.
• Invalid session rehydrates safely to logged-out state.
• Components consume auth store without duplicate local auth state.
• Store shape and actions are documented.