[FEATURE] Ability to choose Entra tenant for signed-in user

## Feature Summary

When logging-in with an Entra user, the default tenant is used and can't be changed.

## Motivation

In some cases an Entra user may be a member of more than one tenant and need to access clusters across tenants.

## Detailed Description

It would be great to be able to choose the tenant at login time, and also switch within the application.
In addition, I foresee some cases where a view of clusters across tenants could be helpful.

## Alternatives Considered

N/A

## Use Cases

Describe the use cases where this feature would be helpful:

1. As a cluster administrator, I want to be able to access my clusters across different tenants so that I can use a single application
2. When using multiple tenants within an organisation, I need to be able to managed my clusters across the different tenants within the application

## Implementation Ideas

An option to choose tenant when logging in, and/or ability to switch between tenants once signed-in.

## Additional Context

N/A

## Acceptance Criteria

- [ ] Allow tenant to be chosen at sign-in time
- [ ] Allow switch of tenant once user is signed-in

## Checklist

- [x] I have searched for existing feature requests
- [x] I have provided clear use cases
- [x] I have considered alternatives
- [x] This feature aligns with the project's goals

The issue names no files or tests; start by locating the Entra sign-in flow and the code that loads and stores the active cluster tenant. Define how tenant selection at sign-in and switching after sign-in should work, then verify both acceptance criteria with coverage for accessing clusters across tenants.

Written by the indexing model from the issue text.