fix(refactor): restore Windows build on backend-pilot

- Re-export `build_mini_player_window` from `lib_commands::ui` so the
  Windows-only pre-create call in `lib.rs` resolves through the
  `use lib_commands::*` glob (was lost during the M5 split).
- Gate the `is_tiling_wm` import in `ui::mini` and its re-export in
  `lib_commands::sync` behind `cfg(target_os = "linux")`, the only
  platform that actually consults it.

src-tauri/src/lib_commands/sync/mod.rs

@@ -5,7 +5,9 @@ pub(crate) use tray::{
     toggle_tray_icon,
 };
 // Internal helpers consumed elsewhere in the shell crate:
-pub(crate) use tray::{is_tiling_wm, stop_audio_engine, try_build_tray_icon};
+pub(crate) use tray::{stop_audio_engine, try_build_tray_icon};
+#[cfg(target_os = "linux")]
+pub(crate) use tray::is_tiling_wm;
 
 // Sync commands now live in `psysonic_syncfs::sync::*` — invoke_handler!
 // in lib.rs registers them with the full path so Tauri's `__cmd__*` magic