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/pubspec.yaml

@@ -36,6 +36,15 @@ dependencies:
     git:
       url: https://git.senkochan.moe/Foxyz/leaf_widgets.git
       ref: v0.1.0
+  # App sounds (sfx/, platform/audio/): SoLoud + miniaudio, built from
+  # source by its build hook (needs only a C++ toolchain).
+  flutter_soloud: ^5.2.1
+# The sounds are WAVs made in memory: no Ogg/Opus/FLAC codecs, so the
+# build hook downloads and bundles no Xiph libraries.
+hooks:
+  user_defines:
+    flutter_soloud:
+      no_xiph_libs: true
 dev_dependencies:
   flutter_test:
     sdk: flutter