ui : reuse the conversation load read for sibling info
Opening a conversation read every message from the database twice: once
in loadConversation for the active path, once in ChatMessages for the
sibling map. Hand the freshly read array over once so the chat screen
builds sibling info from it, and set the conversation and its messages
in one sync block so effects never see the new conversation paired with
the previous one's messages.
Assisted-by: pi:zai-org/GLM-5.3
