Authentication fixes:
- Add isAuthenticated() method to ApiClient to check token presence
- Add useEffect in AdminLayout to redirect to login if no token found
- Fix logout to not fail when token is expired/invalid (401)
- Properly handle logout errors by clearing tokens locally
- Clear tokens and redirect to /login on 401 refresh token failure
UI/UX fixes:
- Replace all deprecated MUI Grid v1 with CSS Grid layout
- Remove Grid import since it's no longer used
- Fix Grid warnings: item, xs, sm, md props deprecated in MUI v7
- Use responsive CSS Grid with gridTemplateColumns for all layouts
Security improvements:
- Check authentication status on every page load
- Auto-redirect to login if no valid session exists
- Handle expired tokens gracefully without breaking logout flow
