Better handling of newly saved conversations

When a new conversation is created in the chat view's
`persistConversation`, we now set `rootMessages` appropriately.

pkg/tui/views/chat/update.go

@@ -166,6 +166,9 @@ func (m Model) Update(msg tea.Msg) (Model, tea.Cmd) {
 	case msgConversationPersisted:
 		m.conversation = msg.conversation
 		m.messages = msg.messages
+		if msg.isNew {
+			m.rootMessages = []models.Message{m.messages[0]}
+		}
 		m.rebuildMessageCache()
 		m.updateContent()
 	case msgMessageCloned: