Sounds play on flutter_soloud instead of three mpv players

SoundEngine (platform/audio) opens the device once (2 s after start,
or on the first sound), loads each synthesized sound into memory (no
temp files), plays overlapping voices and is disposed on window close,
before leaving the server. The SfxService interface and its callers
are unchanged; loudness matches the old mpv path (v^6). WAV only, so
no codec libraries are bundled and no build packages are needed.

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

apps/desktop/lib/main.dart

@@ -73,6 +73,8 @@ Future<void> main(List<String> args) async {
   // Linux: the launcher entry and icon Wayland needs (release installs).
   unawaited(installDesktopEntry());
   runApp(const WatchTogetherApp());
+  // The sound device opens once the window is up, not on the first sound.
+  Future.delayed(const Duration(seconds: 2), SessionState.instance.sfx.warmUp);
 }
 
 /// Close button / Alt+F4: leave gracefully first (others see "left", a
@@ -88,6 +90,11 @@ class _CloseGuard with WindowListener {
     if (_closing) return;
     _closing = true;
     await memory.save();
+    try {
+      await SessionState.instance.sfx.dispose().timeout(const Duration(milliseconds: 500));
+    } catch (e) {
+      debugPrint('sound shutdown on close failed: $e');
+    }
     try {
       await SessionState.instance.leave().timeout(const Duration(seconds: 2));
     } catch (e) {