Adds a lib target (soulseek_rs_tui) exposing cli/models/persist so
integration tests can drive the persistence pipeline: state and config
survive a simulated restart through the real files, with the incomplete
download flagged for auto-resume. The lib e2e suite gains a
register-then-relogin test against soulfind proving the stored-credential
flow's server assumptions. cargo clippy --all-targets and cargo fmt are
clean.
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.
michel