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/api/api.go

@@ -72,6 +72,14 @@ func (m MessageRole) IsAssistant() bool {
 	return false
 }
 
+func (m MessageRole) IsUser() bool {
+	switch m {
+	case MessageRoleUser, MessageRoleToolResult:
+		return true
+	}
+	return false
+}
+
 func (m MessageRole) IsSystem() bool {
 	switch m {
 	case MessageRoleSystem: