Add LastMessageAt field to conversation

Replaced `LatestConversationMessages` with `LoadConversationList`, which
utilizes `LastMessageAt` for much faster conversation loading in the
conversation listing TUI and `lmcli list` command.

pkg/tui/model/model.go

@@ -13,14 +13,9 @@ import (
 	"github.com/charmbracelet/lipgloss"
 )
 
-type LoadedConversation struct {
-	Conv      conversation.Conversation
-	LastReply conversation.Message
-}
-
 type AppModel struct {
 	Ctx           *lmcli.Context
-	Conversations []LoadedConversation
+	Conversations conversation.ConversationList
 	Conversation  *conversation.Conversation
 	Messages      []conversation.Message
 	Model         string
@@ -89,22 +84,6 @@ func (m *AppModel) NewConversation() {
 	m.ApplySystemPrompt()
 }
 
-func (m *AppModel) LoadConversations() (error, []LoadedConversation) {
-	messages, err := m.Ctx.Conversations.LatestConversationMessages()
-	if err != nil {
-		return fmt.Errorf("Could not load conversations: %v", err), nil
-	}
-
-	conversations := make([]LoadedConversation, len(messages))
-	for i, msg := range messages {
-		conversations[i] = LoadedConversation{
-			Conv:      *msg.Conversation,
-			LastReply: msg,
-		}
-	}
-	return nil, conversations
-}
-
 func (a *AppModel) LoadConversationMessages() ([]conversation.Message, error) {
 	messages, err := a.Ctx.Conversations.PathToLeaf(a.Conversation.SelectedRoot)
 	if err != nil {