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/src/shell/settings_tab.dart

@@ -18,7 +18,6 @@ import '../screens/home/about_page.dart';
 import '../screens/home/settings_page.dart';
 import '../sfx/sfx.dart';
 import '../sfx/sound_bank.dart';
-import '../sfx/synth_sfx.dart';
 import '../state/session_state.dart';
 import '../state/settings.dart';
 import '../update/update_controller.dart';
@@ -798,10 +797,7 @@ class NotificationsInfo extends StatelessWidget {
 class SoundSettings extends StatelessWidget {
   const SoundSettings({super.key});
 
-  void _preview(SfxKind kind) {
-    final sfx = SessionState.instance.sfx;
-    if (sfx is SynthSfx) unawaited(sfx.preview(kind, readyIndex: 2));
-  }
+  void _preview(SfxKind kind) => unawaited(SessionState.instance.sfx.preview(kind, readyIndex: 2));
 
   @override
   Widget build(BuildContext context) {