Migrate Security (Users/Roles/Permissions) views to the new React UI

### Description In Airflow 3, the core UI (DAGs, Tasks, Datasets, etc.) was completely rewritten in React with a modern design. However, the Security section — Users, Roles, and Permissions management — still renders using the legacy Flask-AppBuilder (FAB) interface. This creates an inconsistent user experience: navigating from the modern DAGs page to Security/Users feels like switching to a completely different application. The FAB views use a different layout, styling, typography, and interaction patterns than the rest of the Airflow 3 UI. ### Use case/motivation As an Airflow administrator at my workplace, I want to manage Users, Roles, and Permissions through the same modern React UI used for the rest of Airflow 3, rather than being redirected to the legacy Flask-AppBuilder interface. Currently, the Security section feels like a completely different application — different layout, styling, and interaction patterns. This is especially jarring for new users who expect a consistent experience. The REST API already provides the necessary CRUD endpoints at `/auth/fab/v1/` (users, roles), so the main work is building React frontend pages using the existing Airflow 3 design system (Chakra UI). Proposed incremental approach: 1. Users list + detail/edit page 2. Roles list + detail/edit page (with permission assignment) 3. Permissions overview page 4. Deprecate/remove legacy FAB security views ### Related issues No directly related issues found. This is a new feature request to complete the UI modernization started in Airflow 3. ### Are you willing to submit a PR? - [x] Yes I am willing to submit a PR! ### Code of Conduct - [x] I agree to follow this project's [Code of Conduct](https://github.com/apache/airflow/blob/main/CODE_OF_CONDUCT.md)