The default TUI path now brings the terminal up first and runs a login
screen whenever no working credentials exist: a pure-state-machine form
(unit tested) driven by an async connect/login attempt on a background
thread. Soulseek registers unknown usernames on login, so the same form
covers registration. On success the username is saved to config.toml and
a form-entered password to the OS keychain; stored credentials skip the
form entirely. Failed logins show the server error and retry instead of
aborting.
