Fix a default model for users when creating a prompt.

Suggestion:
Fix a default model (recommended an OSS model - Mistral) upon creating a new prompt.

Why?

1. Able to unintentionally showcase the use of OSS models which is aligned with our product goal.

2. Reduce the friction of asking the users to choose the model each time they create a prompt.

Some considerations:

1. If we fix for all users, will it create annoyances?

2. If the user creates the second prompt, do we allow users to maintain the last selected model or hard default it to the model we choose?

3. If we fix the model for them and there is a huge spike of users, will it cause server overload on that model?