Indicate if the current client uses this AdGuard Home instance on the dashboard (DNS check à la AdGuardDNS)

Similarly to AdGuard DNS, we want to be able to see if the current client (in this case, browser) uses the current AdGuard Home instance for DNS resolving. This means that the front end must send a check request to a check domain, and the back end must resolve it, perhaps with some information about the client.