- Added new dashboard template (templates/dashboard/users.html) for managing users
- Added routes in main.py: GET /dashboard/users, POST /dashboard/users/add,
  POST /dashboard/users/{id}/edit, POST /dashboard/users/{id}/toggle,
  POST /dashboard/users/{id}/delete
- Added 'Users' link to navigation menu (visible only for admin users)
- Added update_user method to database.py for editing user details

Features:
- Add new users with username, password, and role (user/admin)
- Edit existing user details
- Toggle user active/inactive status
- Delete users