Display generation model in message header and other tweaks

Adjusted `ctrl+t` in chat view to toggle `showDetails` which toggles the
display of system messages, message metadata (generation model), and
tool call details

Modified message selection update logic to skip messages that aren't
shown

pkg/api/conversation.go

@@ -71,14 +71,22 @@ func ApplySystemPrompt(m []Message, system string, force bool) []Message {
 	}
 }
 
-func (m *MessageRole) IsAssistant() bool {
-	switch *m {
+func (m MessageRole) IsAssistant() bool {
+	switch m {
 	case MessageRoleAssistant, MessageRoleToolCall:
 		return true
 	}
 	return false
 }
 
+func (m MessageRole) IsSystem() bool {
+	switch m {
+	case MessageRoleSystem:
+		return true
+	}
+	return false
+}
+
 // FriendlyRole returns a human friendly signifier for the message's role.
 func (m MessageRole) FriendlyRole() string {
 	switch m {