Warn users about inaccessible DNS IPs

[ainar-g]

Issue Details

The current UI doesn't make it clear that the custom DNS address must be accessible by the VPN server itself. So something like 127.0.0.1 won't work.

Proposed solution

Warn users when they enter addresses that cannot or are unlikely to be accessible, such as localhost addresses, locally-served networks, link-local addresses, etc.

Additionally, it would be nice to have a “Test DNS” button in the future.

Alternative solution

No response