Tweaks/cleanups to conversation management in tui

- Pass around message/conversation values instead of pointers where it
makes more sense, and store values instead of pointers in the globally
(within the TUI) shared `App` (pointers provide no utility here).

- Split conversation persistence into separate conversation/message
  saving stages

pkg/tui/views/chat/chat.go

@@ -4,8 +4,8 @@ import (
 	"time"
 
 	"git.mlow.ca/mlow/lmcli/pkg/api"
-	"git.mlow.ca/mlow/lmcli/pkg/provider"
 	"git.mlow.ca/mlow/lmcli/pkg/conversation"
+	"git.mlow.ca/mlow/lmcli/pkg/provider"
 	"git.mlow.ca/mlow/lmcli/pkg/tui/model"
 	"github.com/charmbracelet/bubbles/cursor"
 	"github.com/charmbracelet/bubbles/spinner"
@@ -20,10 +20,8 @@ type (
 	// sent when a new conversation title generated
 	msgConversationTitleGenerated string
 	// sent when the conversation has been persisted, triggers a reload of contents
-	msgConversationPersisted struct {
-		conversation *conversation.Conversation
-		messages     []conversation.Message
-	}
+	msgConversationPersisted conversation.Conversation
+	msgMessagesPersisted []conversation.Message
 	// sent when a conversation's messages are laoded
 	msgConversationMessagesLoaded struct {
 		messages     []conversation.Message
@@ -35,7 +33,7 @@ type (
 	// sent on each chunk received from LLM
 	msgChatResponseChunk provider.Chunk
 	// sent on each completed reply
-	msgChatResponse *conversation.Message
+	msgChatResponse conversation.Message
 	// sent when the response is canceled
 	msgChatResponseCanceled struct{}
 	// sent when results from a tool call are returned