Release v3.6: rebuild the self-updater (in-app, rollback-safe); warnings to front

Replace the generated cmd.exe + robocopy update helper — which silently
failed on some machines — with an in-app C# installer:
  * Stage the new version to a per-user temp folder off the install and run
    the new RemSound.exe from there, so nothing in the install is locked by
    the updater itself.
  * Wait for the old process to fully exit (real WaitForExit), then
    back-up-and-swap files in C# with retry + rename-aside; roll the install
    back to the previous version on any failure, so a failed update can never
    leave a half-installed RemSound.
  * Log every step to updater.log; clean up old stages and legacy batch
    artefacts on launch. Removed the old BuildInstallScript batch generator.

Route the "RemSound is already running" dialog and its follow-up message
through ForegroundDialog so they surface in front from a background-relaunched
copy, matching the earlier post-update fix.

Docs: rewrite the readme update sections for the new mechanism, regenerate
MANUAL.md, refresh the About-box changelog and RELEASE_NOTES.

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>3.5.0</Version>
+    <Version>3.6.0</Version>
   </PropertyGroup>
 
   <ItemGroup>