Set default `user`

### Describe the feature or problem you'd like to solve

Please allow a way to set default user. I mostly use my work one but have a personal one too. Hate that it keeps defaulting to most recently added even if I use that one way less.

### Proposed solution

Helps people who balance multiple accounts but with extreme difference in usage.

### Example prompts or workflows

_No response_

### Additional context

_No response_

No file, test, or entry point is named. Start by tracing how the CLI selects among multiple authenticated users and where account preferences are stored or loaded; done means users can choose a default account that remains selected instead of being replaced by the most recently added account.

Written by the indexing model from the issue text.