fix: empty changeset must serialize as [] not null (white-screen after snapshot)

toChangesetResponse initialises updates/prunes/readOnly so a zone matching
its template exactly (e.g. right after 'create template from zone') marshals
arrays, not null. DiffView/DomainDiffPage also normalise null defensively.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01BwxdSt4reTm7Dj1oxRvpP3

web/src/pages/DomainDiffPage.tsx

@@ -41,8 +41,8 @@ export function DomainDiffPage() {
   const pruneCheckboxId = useId()
 
   const changeset = check.data
-  const hasPrunes = (changeset?.prunes.length ?? 0) > 0
-  const hasUpdates = (changeset?.updates.length ?? 0) > 0
+  const hasPrunes = (changeset?.prunes?.length ?? 0) > 0
+  const hasUpdates = (changeset?.updates?.length ?? 0) > 0
   const pruneWarning = applyPrunes && hasPrunes
   const recordList = zoneRecords.data ?? []