Introduces a `Config` struct to manage client settings, including server
URL and API key.
This configuration is stored in a TOML file following OS-standard
conventions.
The `directories` crate is used to determine the correct path for the
configuration file across different operating systems.
The `thiserror` crate is used for custom error handling related to
configuration loading and saving.
Added unit tests to verify the configuration loading and saving logic.
Brummel
c441377749