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/lib.rs

@@ -1,6 +1,7 @@
 //! Library surface of the TUI binary, exposed for integration tests.
 //! The binary itself compiles these modules directly (see `main.rs`).
 
+pub mod api;
 pub mod cli;
 pub mod models;
 pub mod persist;