test: persistence e2e and registration-relogin e2e; lint clean

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.

soulseek-rs-lib/tests/e2e.rs

@@ -417,6 +417,30 @@ fn wrong_password_is_rejected() {
     );
 }
 
+#[test]
+fn registered_username_can_relogin_with_same_password() {
+    let server = server_or_skip!();
+
+    // The TUI's stored-credentials flow depends on this pair of server
+    // behaviors: a fresh username is registered by simply logging in, and a
+    // later session (a "restart") with the same credentials is accepted.
+    let user = "e2e_relogin_user";
+    let mut first = Client::with_settings(server.settings(user, "pw-123"));
+    first.connect().expect("connect (registering login)");
+    assert!(
+        first.login().expect("registering login"),
+        "a fresh username should be auto-registered"
+    );
+    drop(first);
+
+    let mut second = Client::with_settings(server.settings(user, "pw-123"));
+    second.connect().expect("connect (relogin)");
+    assert!(
+        second.login().expect("relogin"),
+        "the same credentials must log in again after a restart"
+    );
+}
+
 #[test]
 fn two_clients_can_be_logged_in_together() {
     let server = server_or_skip!();