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)