🐛 fix: preserve scroll position when switching sessions

When switching between topics/sessions, the displayMessages were cleared
to empty before replaceMessages was called, causing the virtual list to
reset its scroll position to the bottom.

Now replaceMessages is called directly without clearing first, preserving
the scroll position. Empty state is only set when no messages are available.

Closes #4984

src/features/Conversation/StoreUpdater.tsx

@@ -85,8 +85,6 @@ const StoreUpdater = memo<StoreUpdaterProps>(
         // when calling onMessagesChange (otherwise writes to the old topic key)
         storeApi.setState({
           context,
-          dbMessages: messages ?? [],
-          displayMessages: [],
           messagesInit: false,
         });
 
@@ -94,6 +92,8 @@ const StoreUpdater = memo<StoreUpdaterProps>(
         if (messages) {
           storeApi.getState().replaceMessages(messages);
           storeApi.setState({ messagesInit: true });
+        } else {
+          storeApi.setState({ dbMessages: [], displayMessages: [] });
         }
       }
     }, [contextKey]); // eslint-disable-line react-hooks/exhaustive-deps