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

internal/api/dto.go

@@ -72,7 +72,15 @@ func toRecordView(d diff.RecordDiff) recordView {
 }
 
 func toChangesetResponse(cs diff.Changeset) changesetResponse {
-	resp := changesetResponse{}
+	// Initialise the slices so an empty changeset (e.g. a zone that exactly
+	// matches its template right after a snapshot) marshals to [] rather than
+	// null — a nil slice becomes JSON null, which crashes clients that call
+	// .length/.map on the field.
+	resp := changesetResponse{
+		Updates:  []recordView{},
+		Prunes:   []recordView{},
+		ReadOnly: []recordView{},
+	}
 	for _, d := range cs.Updates() {
 		resp.Updates = append(resp.Updates, toRecordView(d))
 	}