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>

docs/api.md

@@ -411,7 +411,9 @@ meantime stops it (and gives a `disabled_conflict` warning until then).
 `PUT /rrsets` makes the RRset exactly the given data: a disabled record
 with one of the values is enabled, other disabled records are left alone.
 Disabling a record that is already disabled is a 409; with `missing_ok=1`
-(by match) it's "nothing to do".
+(by match) it's "nothing to do". A zone update that leaves the zone as it
+is (the same SOA, TTL, comment...) changes nothing, so applying it is
+"nothing to do" and staging it is "nothing to change".
 
 ### Views
 
@@ -510,7 +512,7 @@ written in Jarl's fixed order (docs/concepts.md).
 | GET | `/changesets` | `?status=open\|applied\|discarded&mine=1&limit=100&before=ID` |
 | POST | `/changesets` | `{"description": "...", "ops": [...], "apply": false}`: create (and optionally stage or apply) |
 | GET / PATCH | `/changesets/{id}` | PATCH `{"description": "..."}` |
-| POST | `/changesets/{id}/ops` | `{"ops": [Operation...]}`: stage more operations |
+| POST | `/changesets/{id}/ops` | `{"ops": [Operation...]}`: stage more operations; 400 "nothing to change" if they change nothing |
 | DELETE | `/changesets/{id}/ops/{seq}` | remove one staged operation |
 | GET | `/changesets/{id}/preview` | dry run of an open changeset: problems, check messages, diff |
 | POST | `/changesets/{id}/apply` | `{"dry_run": false, "force": false}` |