Registrar: only for zones that are a registered domain

With porkbun.domains empty the Registrar tab was offered for every zone
and always failed for subdomain zones, reverse zones and private names.
dnsname.RegisteredDomain (public suffix list) now decides; the others
get a message naming what delegates them.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

CHANGELOG.md

@@ -33,6 +33,14 @@ All notable changes to Jarl are documented here. The format follows
 
 ### Fixed
 
+- The Registrar tab (and `jarl zone registrar`, and the DNSSEC page's
+  registrar button) was offered for every zone when `porkbun.domains` is
+  empty, and always failed for subdomain zones (`lab.adbraun.lol`),
+  reverse zones and private names (`home.lan`): Porkbun only knows
+  registered domains. Only zones that are a registered domain (by the
+  public suffix list) get it now; the others say what delegates them
+  instead.
+
 - Auto-PTR in linked zones changed PTR records in every linked view, not
   just where the address record is. A record local to one view got PTR
   records in the other views too; deleting or changing the address in