Handle refresh tokens more securely

From Aidan's comments in #6...

> We may want to store refresh tokens more securely in the future. This was a quick-and-dirty way of doing it at the time, but I'm not sure saving the tokens in plain text on the hard drive is following good security practices...

Look for where refresh tokens are currently stored, likely in a plain text file. Research secure storage options in Rust, such as using the OS keychain or encrypted files. Determine the project's authentication flow to integrate the new storage method safely.

Escrita pelo modelo de indexação a partir do texto da issue.