fix(topics): default message list to newest-first ordering

The message table defaulted to `sorting: []`, so rows rendered in raw
backend stream order (per-partition arrival, oldest-first within a
partition) instead of newest-first. A newest-first default existed in the
original 2019 table (offset:descending) but was dropped in the migration
to the nuqs/TanStack table; the only surviving newest-first sort was gated
behind continuous pagination.

For the "Newest" (last N / EndMinusResults) view, sort by timestamp
descending with offset descending as a tiebreaker. Offset isn't global
across partitions, so timestamp is the primary key. This applies only when
the user hasn't chosen an explicit column sort, so header sorting still
wins; continuous pagination (which forces sorting to []) also lands here.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: jvorcak

frontend/src/components/pages/topics/Tab.Messages/index.tsx

@@ -572,11 +572,14 @@ export const TopicMessageView: FC<TopicMessageViewProps> = (props) => {
       })
     : messages;
 
-  // For continuous pagination, just use the filtered messages directly
-  // We don't use placeholders as they cause page content to shift
+  // Default ordering for the "Newest" (last N) view: show messages newest-first, sorted by
+  // timestamp descending with offset descending as a tiebreaker. Offset isn't global across
+  // partitions, so timestamp is the primary key. This applies unless the user has picked an
+  // explicit column sort (sorting.length > 0), in which case the table's sorted row model wins.
+  // Continuous pagination forces sorting to [] and disables column sorting, so it always lands here.
   const filteredMessages =
-    continuousPaginationEnabled && startOffset === PartitionOffsetOrigin.EndMinusResults
-      ? [...baseFilteredMessages].sort((a, b) => b.timestamp - a.timestamp)
+    startOffset === PartitionOffsetOrigin.EndMinusResults && sorting.length === 0
+      ? [...baseFilteredMessages].sort((a, b) => b.timestamp - a.timestamp || b.offset - a.offset)
       : baseFilteredMessages;
 
   // Convert @computed activePreviewTags to useMemo