FEAT: Select active account

[wakonig]

As a beamline scientist,
I want to change the currently selected BEC account for a specific deployment,
so that I can operate BEC with the correct permissions and identity without redeploying or reconfiguring services.

Acceptance criteria

• The UI shows the currently active account for the selected deployment
• Users can switch to another authorized account
• Changes take effect immediately and are visible in the deployment state
• Unauthorized accounts cannot be selected
• Only users with sufficient permissions can modify the account