Integrate role retrieval into AuthContext for RBAC-ready frontend state

[alizahh-7]

This PR integrates user role retrieval into the authentication context to prepare the frontend for role-based access control (RBAC).

Changes

• Adds role state to AuthContext
• Fetches the user role after authentication using fetchUserRole
• Stores the role in the global authentication context
• Exposes role through the useAuth() hook for use across the UI

Why

Role information is required for implementing role-based UI behavior and RBAC features.
By retrieving and storing the role in AuthContext, the frontend can later conditionally render components and restrict functionality based on user permissions.

Files Modified

ui/context/AuthContext.tsx

Notes

• Uses the existing fetchUserRole utility from ui/server/fetchUser.ts
• No backend changes were required

Future Use

This prepares the frontend architecture for upcoming RBAC-related features.