Option to edit system prompt before starting a new chat

Presently, when starting a new chat without selecting an agent, there's no option to modify the default system prompt. This forces you to override it manually in the first message, which is inconvenient, especially because the existing "o3" prompt instructs the assistant to use "$$" around mathematical content. As a result, sometimes the assistant incorrectly uses "$$" around code snippets, breaking the formatting.