Release v3.2: update sound cue + single-instance bring-to-front fix

- New "Update sound" cue (CueId.Update / update.wav). Plays just before an
  update starts installing, on every path (manual, background-silent,
  startup-silent), so a silent background update gives an audible heads-up
  before RemSound closes to restart. Per-profile mute + custom-sound override
  in Preferences, same infrastructure as the other cues:
    * Profile.EnableUpdateCue + RemSoundSettingsStore Load/Save + round-trip
    * MainForm updateSound field, TryLoadCueSound, play in InstallUpdateAsync
      gated by LoadEnableUpdateCue
    * PreferencesDialog "Update sound" CueRow + ResolveCueFilePath mapping
    * update.wav shipped in sounds\ via csproj Content
- Single-instance "switch to the running copy" now actually brings the window
  to the front. The second copy grants the running copy foreground rights via
  AllowSetForegroundWindow before signalling, and lingers briefly so it can
  raise itself before we exit — without this, Windows' foreground lock left
  the running window only flashing in the taskbar (Ed's report).
- Docs: About box v3.2 block, RELEASE_NOTES.md, manual cue section (now seven
  cues, with the update cue described), MANUAL.md regenerated.
- Version bumped 3.1.3 -> 3.2.0.

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>3.1.3</Version>
+    <Version>3.2.0</Version>
   </PropertyGroup>
 
   <ItemGroup>
@@ -90,6 +90,13 @@
       <Link>sounds\profile.wav</Link>
       <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
     </Content>
+    <!-- update.wav (2026-05-31): plays just before an update starts installing, so a silent
+         background update still gives an audible heads-up. Per-profile mute + custom-sound
+         override available in Preferences, same as the other cues. -->
+    <Content Include="..\..\sounds\update.wav" Condition="Exists('..\..\sounds\update.wav')">
+      <Link>sounds\update.wav</Link>
+      <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
+    </Content>
     <!-- User manual. F1 anywhere in the app opens this via the user's default browser
          (HelpLauncher.OpenManual). The PreserveNewest mode means a fresh publish overwrites
          the published copy whenever the source is newer; manually-edited copies inside