Release v4.3.1: quieter default update sound

Both update cue variants ("update 1.wav" / "update 2.wav") replaced with a
gentler mix that signals an incoming update without interrupting work mid-flow.
About-box note added (tidied wording); csproj <Version> bumped to 4.3.1. No code
changes.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/RemSound.App/RemSound.App.csproj

@@ -14,7 +14,7 @@
          tag_name on the latest GitHub release; bump it on every public release. The
          AssemblyVersion / FileVersion default to this value, and Assembly.GetName().Version
          is what the About dialog and the updater both read. -->
-    <Version>4.3</Version>
+    <Version>4.3.1</Version>
   </PropertyGroup>
 
   <ItemGroup>