Desktop build plays music and sound through the system sound output; desktop build documented

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Cargo.toml

@@ -21,3 +21,4 @@ opt-level = 1
 # desktop builds: system file dialog (xdg portal on Linux) and sound output
 [target.'cfg(not(target_arch = "wasm32"))'.dependencies]
 rfd = { version = "0.17", default-features = false, features = ["xdg-portal"] }
+cpal = "0.18"