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>

internal/dnsname/dnsname_test.go

@@ -249,3 +249,23 @@ func TestPrefixFromReverse(t *testing.T) {
 		}
 	}
 }
+
+func TestRegisteredDomain(t *testing.T) {
+	for name, want := range map[string]string{
+		"adbraun.lol.":          "adbraun.lol",
+		"lab.adbraun.lol.":      "adbraun.lol",
+		"Example.CO.UK":         "example.co.uk",
+		"co.uk.":                "",
+		"lol.":                  "",
+		"10.0.10.in-addr.arpa.": "",
+		"d.f.ip6.arpa.":         "",
+		"home.arpa.":            "",
+		"nas.home.lan.":         "",
+		"":                      "",
+	} {
+		got, ok := RegisteredDomain(name)
+		if got != want || ok != (want != "") {
+			t.Errorf("RegisteredDomain(%q) = %q, %v; want %q", name, got, ok, want)
+		}
+	}
+}