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/AboutDialog.cs

@@ -20,6 +20,16 @@ internal sealed class AboutDialog : Form
     /// updates" path.</summary>
     private const string ReleaseNotes =
         """
+        RemSound v4.3.1
+
+        A gentler update sound. The cue that plays when
+        an update is on its way is now quieter by default
+        — enough to let you know it's coming, without
+        breaking your concentration while you're working.
+        As before, you can choose your own update sound,
+        or switch it off, on the Audio cues tab in
+        Preferences.
+
         RemSound v4.3
 
         Two additions for screen-reader users, and a