Remove AppChannels, use separate receivers

Refactor App::run to accept individual Receiver arguments instead of an
AppChannels struct. Delete the AppChannels definition and its re‑export,
and update main.rs to create each channel manually and pass the
receivers
to App::run. Rename the CLI field `sound_track` to `soundtrack` and
adjust
all references. Clean up imports and formatting accordingly.

src/app/threads/mod.rs

@@ -1,9 +1,7 @@
-mod app_channels;
 mod audio;
 mod events;
 mod tick;
 
-pub use app_channels::AppChannels;
 pub use audio::{AudioCmd, SoundrackParts, Soundtrack, handle_audio};
 pub use events::handle_ct_events;
 pub use tick::handle_tick_event;