feat: add load earlier support to chat timeline

Adds a load older button when earlier history is available
Preserves scroll position while older messages are loaded
Shows date-grouped messages with clearer per-message timestamps

packages/ui/src/components/chat/ChatContainer.tsx

@@ -993,6 +993,9 @@ export const ChatContainer: React.FC<ChatContainerProps> = ({ autoOpenDraft = tr
                 onScrollToMessage={timelineController.scrollToMessage}
                 onScrollByTurnOffset={navigation.scrollByTurnOffset}
                 onResumeToLatest={resumeToLatestInstant}
+                canLoadEarlier={timelineController.historySignals.canLoadEarlier}
+                isLoadingEarlier={timelineController.isLoadingOlder}
+                onLoadEarlier={handleLoadOlderClick}
             />
         </div>
     );