ui: highlight the current tenant in the DB console UI

[knz]

Opening the DB console on the system tenant will become less common once we push most users to use a secondary tenant.

We're expecting that system tenant console will only be needed in very rare cases; and that if a user finds themselves there, it will be more often by accident than intentionally.

So we should highlight clearly (e.g. with a banner, and perhaps with a color theme) that the system tenant is a non-standard configuration and the user should be more attentive.

Jira issue: CRDB-22179
Epic CRDB-34076