Role-Based UI Handling

[grantfox-oss]

Description:
Show or hide UI elements based on the user's role (user or admin).

Tasks:

• Create a src/hooks/useRole.js custom hook that returns { isAdmin, isUser } based on selectCurrentUser
• Hide the "Create Campaign" button for admin users
• Show the "Admin Panel" link in the Navbar only for admin users
• Conditionally render "Edit" and "Delete" buttons on campaign cards only for the campaign owner
• Create a <AdminRoute> component that redirects non-admins to /dashboard

Acceptance Criteria:
Admin-only UI is hidden from regular users; owner-only actions are hidden from non-owners.