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.test.tsx

@@ -26,3 +26,17 @@ test("marks read-only records", () => {
   render(<DiffView changeset={cs} />)
   expect(screen.getByText(/NS/)).toBeInTheDocument()
 })
+
+test("does not crash when changeset fields are null", () => {
+  // An empty changeset from an older/edge backend can arrive with null slices
+  // instead of []. DiffView must normalise them, not blow up on .length/.map.
+  const nullish = {
+    updates: null,
+    prunes: null,
+    readOnly: null,
+    inSyncCount: 5,
+  } as unknown as ChangesetResponse
+  render(<DiffView changeset={nullish} />)
+  expect(screen.getByText(/5/)).toBeInTheDocument()
+  expect(screen.getByText(/in sync/)).toBeInTheDocument()
+})

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">

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 ?? []