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/AboutDialog.cs

@@ -20,6 +20,34 @@ internal sealed class AboutDialog : Form
     /// updates" path.</summary>
     private const string ReleaseNotes =
         """
+        RemSound v3.2
+
+        A new audio cue, plus the reliability work from
+        the recent updates, rolled into one release.
+
+        New "Update sound" cue: RemSound now plays a short
+        sound just before it closes to install an update —
+        for both manual updates and silent background ones.
+        So a silent update no longer takes you by surprise;
+        you hear it coming. Like every cue it's per-profile,
+        can be muted, and you can swap in your own sound,
+        all from File → Preferences → Audio cue sounds.
+
+        Only one copy at a time: RemSound refuses to run as
+        two copies at once. Open it while it's already
+        running and it offers to switch you to the copy
+        that's already going (now reliably bringing that
+        window to the front, even from the system tray), or
+        — if that copy is stuck — to force it closed and
+        start fresh.
+
+        Updates can't be blocked or doubled up: nothing can
+        get in the way of an update's restart any more, and
+        a single copy can't kick off two installs at once.
+
+        Locked profiles stay locked: marking a profile as
+        read-only now survives saving it.
+
         RemSound v3.1.3
 
         An important reliability fix. After an update, a