- Move commands/player.rs to commands/player/mod.rs (folder module).
- Extract the 5 self-contained remote-session control commands
(remote_play_on_session/send_command/session_seek/set_volume/toggle_mute)
into commands/player/remote.rs, re-exported via `pub use remote::*` so the
command names remain at commands::player::* and the invoke_handler is unchanged.
No behavior change; establishes the submodule split pattern for the oversized
command file.