phsl / new-api

- Extend handleMessageReset condition to include 'system' role messages
- Allow system messages to trigger regeneration like assistant messages
- Fix disabled retry button issue when message role is switched to system
- Maintain consistent user experience across different message roles

Previously, when an assistant message was switched to system role,
the retry button became non-functional. This change ensures that
system messages can be regenerated by finding the previous user
message and resending it, maintaining feature parity with assistant
messages.