Improve the basic playground default message

[Shaunwei]

At least make it something like

[
  {
    "role": "system",
    "content": "You're an assistant, please answer the following questions."
  },
  {
    "role": "user",
    "content": "what is ReByte?"
  }
]

It should try to be default right

[Shaunwei]

another issue, it doesn't persist user input. if switch tab the input text will be cleared

[y1guo]

done