Zone settings: no empty changes, keep an outside name server absolute

- A zone update that changes nothing stages nothing: saving a zone's
  settings unchanged says "Nothing changed" instead of staging an empty
  change. Staging operations that change nothing is a 400 "nothing to
  change".
- The settings form showed the SOA MNAME without its final dot, which the
  server reads as relative to the zone, so saving wg.adbraun.lol turned
  ns1.adbraun.lol. into ns1.adbraun.lol.wg.adbraun.lol. Names outside the
  zone keep the dot, and MNAME is only sent when edited.

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

internal/engine/expand.go

@@ -5,6 +5,7 @@ import (
 	"encoding/json"
 	"fmt"
 	"net/netip"
+	"reflect"
 	"slices"
 	"strings"
 	"time"
@@ -193,6 +194,11 @@ func (x *expander) expandOne(req Request) ([]model.Op, error) {
 			if in.LinkWith != "" {
 				return nil, errf(KindBadRequest, "link_with is for new zones; link existing ones with zone.link (jarl zone link)")
 			}
+			// An update that leaves the zone as it is stages nothing (a
+			// settings form saved without changes).
+			if zoneUnchanged(x.st, z, &in) {
+				return nil, nil
+			}
 		}
 		return ops, nil
 	case OpRecordCreate:
@@ -1555,3 +1561,17 @@ func firstNonEmptyStr(a, b string) string {
 func notLinked(st *model.State, z *model.Zone) error {
 	return errf(KindBadRequest, "zone %s isn't linked with other views, so there is nothing to share its records with (link it first: jarl zone link)", zoneDesc(st, z))
 }
+
+// zoneUnchanged reports whether an update would leave the zone as it is.
+// Errors are left for applying the update to report.
+func zoneUnchanged(st *model.State, z *model.Zone, in *ZoneInput) bool {
+	if len(in.Nameservers) > 0 || len(in.Records) > 0 || in.CloneFrom != 0 || len(in.Options) > 0 {
+		return false
+	}
+	c := st.Clone()
+	n := c.Zones[z.ID]
+	if err := setZoneFields(c, n, in, false); err != nil {
+		return false
+	}
+	return reflect.DeepEqual(n, z)
+}