Prompt user for OpenAI API key
Closes #34 (closed)
This PR prompts the user to enter an API key for OpenAI when opening the app for the first time. They can choose to add it then or later when they use the AI-assisted features.
- Using the AI-assisted features is blocked until they have entered an API key
- We store the API key on the user's system in a preferences file (see !18 (merged)) so when opening the app in the future, they are not prompted to enter an API key if they have already done so