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/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>4.5</Version>
+    <Version>4.6</Version>
   </PropertyGroup>
 
   <ItemGroup>