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/engine_test.go

@@ -462,3 +462,39 @@ func TestParseWarningsReachTheResult(t *testing.T) {
 		t.Errorf("ops = %+v", res.Ops)
 	}
 }
+
+func TestUnchangedZoneUpdateStagesNothing(t *testing.T) {
+	env := newEnv(t)
+	setupZone(env)
+	env.submit(env.admin, NewRequest(OpZoneCreate, 0, ZoneInput{Name: model.Ptr("wg.example.com")}))
+	z := env.state().FindZone("wg.example.com.", 0)
+	if z.SOA.MName != "ns1.example.com." {
+		env.submit(env.admin, NewRequest(OpZoneUpdate, z.ID, ZoneInput{SOA: &SOAInput{MName: model.Ptr("ns1.example.com.")}}))
+		z = env.state().FindZone("wg.example.com.", 0)
+	}
+	cs, err := env.e.CreateChangeset(env.ctx, env.admin, "web", "")
+	if err != nil {
+		t.Fatal(err)
+	}
+	// The settings form saved as it was.
+	same := ZoneInput{Comment: model.Ptr(z.Comment), DefaultTTL: model.Ptr(z.DefaultTTL), SerialPolicy: model.Ptr(z.SerialPolicy),
+		SOA: &SOAInput{RName: model.Ptr("hostmaster@example.com"), Refresh: model.Ptr(z.SOA.Refresh), Retry: model.Ptr(z.SOA.Retry),
+			Expire: model.Ptr(z.SOA.Expire), Minimum: model.Ptr(z.SOA.Minimum)}}
+	if z.SOA.RName != "hostmaster.example.com." {
+		same.SOA.RName = nil
+	}
+	_, err = env.e.Stage(env.ctx, cs.ID, []Request{NewRequest(OpZoneUpdate, z.ID, same)}, env.admin)
+	if e, ok := AsError(err); !ok || e.Kind != KindBadRequest || e.Message != "nothing to change" {
+		t.Fatalf("unchanged update: %v", err)
+	}
+	if e := env.submitErr(env.admin, NewRequest(OpZoneUpdate, z.ID, same)); e.Message != "nothing to do" {
+		t.Errorf("unchanged update applied: %+v", e)
+	}
+	// A real change still stages, and an absolute MNAME stays absolute.
+	if _, err := env.e.Stage(env.ctx, cs.ID, []Request{NewRequest(OpZoneUpdate, z.ID, ZoneInput{Comment: model.Ptr("the VPN")})}, env.admin); err != nil {
+		t.Fatal(err)
+	}
+	if got := env.state().FindZone("wg.example.com.", 0).SOA.MName; got != "ns1.example.com." {
+		t.Errorf("mname = %q", got)
+	}
+}