fix(refactor): cfg-gate two items so macOS build is warning-clean (#530)

Mac smoke build surfaced 5 dead-code / unused-import warnings, all
cfg-leaks of items that are conditionally compiled on Windows + Linux:

- `psysonic-audio::power_resume` is consumed by `power_notify_win` and
  `power_notify_linux` only — `register_post_sleep_audio_recovery`
  intentionally falls through to a no-op on macOS (the generic device
  watcher covers the resume case there). Gate the module declaration
  to `#[cfg(any(target_os = "windows", target_os = "linux"))]`.

- `lib_commands::ui::build_mini_player_window` is re-exported for the
  Windows-only pre-create path in `lib.rs:setup` (other platforms
  create the mini-player webview lazily on first invoke). Gate the
  re-export to `#[cfg(target_os = "windows")]` so non-Windows builds
  don't warn on an unused import.

Both are non-functional — the items themselves are already correctly
scoped via cfg in their consumers; only the declarations / re-exports
were missing the matching gate.

src-tauri/crates/psysonic-audio/src/lib.rs

@@ -20,6 +20,7 @@ pub mod radio_commands;
 pub mod transport_commands;
 mod device_watcher;
 mod engine;
+#[cfg(any(target_os = "windows", target_os = "linux"))]
 mod power_resume;
 #[cfg(target_os = "windows")]
 mod power_notify_win;