Warn users about inaccessible DNS IPs

### 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_