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/state/session_state.dart

@@ -160,7 +160,8 @@ class SessionState {
   final route = signal(AppRoute.home);
   final recents = signal<List<RecentServer>>(const []);
   final status = signal(const HealthStatus(HealthSeverity.info, 'idle'));
-  final SfxService sfx = SynthSfx(
+  /// Owns the app's audio device ([SynthSfx.dispose] on close).
+  final SynthSfx sfx = SynthSfx(
     volume: () => AppSettings.instance.soundVolume.peek(),
     // A nudge is an explicit poke: it ignores its switch (not the volume).
     enabled: (k) => k == SfxKind.nudge || !AppSettings.instance.soundsOff.peek().contains(k.name),