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/conversation/conversation.go

@@ -17,6 +17,7 @@ type Conversation struct {
 	SelectedRootID *uint
 	SelectedRoot   *Message  `gorm:"foreignKey:SelectedRootID"`
 	RootMessages   []Message `gorm:"-:all"`
+	LastMessageAt  time.Time
 }
 
 type MessageMeta struct {