ui: [FEATURE] Disable DC switching when Peering is enabled

### Description

This should utilize the same configuration that is being created to enable Peering mode that would be also used for #13269

This work has a blocking dependency on the backend implementing the config.

### Requirements

1. When the Peering configuration is enabled, the datacenters dropdown is disabled and instead displays the datacenter for which the login token belongs to
2. Users must use separate tokens to log into/view separate datacenters (and therefore to establish a Peering between separate datacenters)
3. No changes to UI routing or URL schemes at this time