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/components/DiffView.tsx

@@ -137,11 +137,13 @@ export function DiffView({
   changeset: ChangesetResponse
   footerExtra?: ReactNode
 }) {
+  // Defensive: a field may arrive as null (e.g. a nil slice from an older
+  // backend) — normalise to [] so Section never calls .length/.map on null.
   return (
     <div className="flex flex-col gap-6">
-      <Section tone="update" records={changeset.updates} />
-      <Section tone="delete" records={changeset.prunes} />
-      <Section tone="readonly" records={changeset.readOnly} />
+      <Section tone="update" records={changeset.updates ?? []} />
+      <Section tone="delete" records={changeset.prunes ?? []} />
+      <Section tone="readonly" records={changeset.readOnly ?? []} />
 
       <div className="flex items-center justify-between gap-3 border-t border-border pt-4">
         <div className="flex items-center gap-1.5 text-sm text-muted-foreground">