Release v4.6: follow the Windows default audio device (output + input)

- New "Use Windows default audio device, follows Windows changes" entry at the top of the
  received-output and send-input WASAPI lists. Resolves the current Windows default live,
  re-routes automatically when the default device changes, works alongside specific devices,
  and persists across launches (a follower can't go stale). Optional "untick the others?"
  prompt with a remembered "don't ask again", reset via a new Options item "Reset the default
  audio device prompt".
- Manual updated for the feature, plus a sweep that corrected the now-stale Options-menu
  section (retired Startup-behaviour item, four->five Preferences tabs, missing entries).
  About changelog and RELEASE_NOTES updated; version 4.6.

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

src/RemSound.App/AboutDialog.cs

@@ -20,6 +20,25 @@ internal sealed class AboutDialog : Form
     /// updates" path.</summary>
     private const string ReleaseNotes =
         """
+        RemSound v4.6
+
+        Follow your Windows default audio device.
+
+        Both device lists — the outputs for received
+        sound and the inputs to send — now have a "Use
+        Windows default audio device, follows Windows
+        changes" entry at the top. Tick it and RemSound
+        uses whatever Windows is currently using, and
+        switches with it on its own: make a headset your
+        default and RemSound follows, no clicking. You
+        can still tick specific devices alongside it.
+
+        When you turn it on, RemSound offers to untick
+        the other devices so you use only the default;
+        if you hide that question with "Don't ask
+        again", Options → Reset the default audio device
+        prompt brings it back.
+
         RemSound v4.5
 
         Keyboard shortcuts get easier to manage.