Add a dedicated DNS‐resolution health check and admin warning for DNS failures

Description Nextcloud currently assumes that the host’s DNS resolver is functional and only surfaces DNS‐related errors indirectly (e.g. “Could not check that your web server can connect to itself”). In environments where DNS is misconfigured or temporarily unreachable, this can lead to severe performance degradation—pages hang until DNS timeouts expire—without any clear indication in the admin UI that DNS is the root cause. I propose adding a dedicated DNS‐resolution health check to the Administration → Overview → Security & setup warnings page that: 1. Attempts to resolve and connect to at least one of the trusted_domains or overwrite.cli.url entries. 2. Optionally performs a lightweight resolution test against a well-known host (e.g. updates.nextcloud.org) to detect broader external DNS issues. 3. Surfaces a clear warning (“Your server’s DNS resolution appears broken: Nextcloud will be very slow or non-functional until this is resolved.”) with guidance on how to fix common D…