Desktop build: settings and progress kept as files, savegames in a saves folder, system file dialog for loading

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

Cargo.toml

@@ -17,3 +17,7 @@ codegen-units = 1
 
 [profile.dev]
 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"] }