refactor(api): put a SessionApi seam between the commands, the TUI, and the client

Commands and the TUI now hold an Arc<dyn SessionApi> instead of an
Arc<Client>. The trait is implemented on Client itself, so the local path
is the same client behind one vtable and no wrapper type exists to keep in
sync. Daemon mode plugs a remote implementation into the same handle.

soulseek-rs/src/ui/main_tui/render.rs

@@ -264,7 +264,7 @@ impl MainTui {
         // it renders the messages and the compose line itself.
         let area = centered_rect(70, 60, frame.area());
         frame.render_widget(ratatui::widgets::Clear, area);
-        render_chat_pane(frame, area, &self.state, self.client.username());
+        render_chat_pane(frame, area, &self.state, &self.client.username());
     }
 
     /// Context shortcuts for the chat-rooms popup.