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/src/ui/main_tui/mod.rs

@@ -88,7 +88,8 @@ impl MainTui {
                         status: soulseek_rs::DownloadStatus::Completed,
                         sender: std::sync::mpsc::channel().0,
                         queue_position: None,
-                        metadata: Default::default(),
+                        metadata: soulseek_rs::types::DownloadMetadata::default(
+                        ),
                     },
                     receiver: None,
                 });