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/models/app_state.rs

@@ -111,6 +111,7 @@ pub struct AppState {
 }
 
 impl AppState {
+    #[must_use]
     pub fn new() -> Self {
         let mut searches_table_state = TableState::default();
         searches_table_state.select(Some(0));
@@ -166,6 +167,7 @@ impl AppState {
     }
 
     #[allow(dead_code)]
+    #[must_use]
     pub fn get_selected_search(&self) -> Option<&SearchEntry> {
         self.selected_search_index
             .and_then(|idx| self.searches.get(idx))