Bump to v1.9.0: fix the robocopy trailing-backslash bug — auto-updater finally works

The install helper's robocopy line was:
  robocopy "{stagingRoot}" "{installDir}" ...
installDir is AppContext.BaseDirectory, which always ends in a directory
separator, so the destination argument was a quoted path ending in a
backslash: "D:\...\publish\". Windows command-line parsing reads the \"
as an escaped quote, so robocopy never received a valid destination,
rejected the command line, and exited 16 (usage error, nothing copied)
instantly. The auto-updater has never worked in any release because of
this — v1.0-v1.2 failed silently, v1.3+ detected the failure and wrote
update-failed.txt but never fixed the robocopy line.

Fix: BuildInstallScript now strips trailing separators —
stagingArg = stagingRoot.TrimEnd('\','/'), installArg likewise — and
the robocopy line uses the trimmed forms. Verified by running the
corrected robocopy against real staged files: exit 3 (success), files
copied.

The broken helper is baked into every shipped build including v1.8, and
the helper is generated by the running version — so v1.8 and earlier
cannot auto-install v1.9. v1.9 must be installed by hand once; from v1.9
onward the updater works.

No wire-format or audio-pipeline changes — v1.5 through v1.9 interoperate.

Co-Authored-By: Claude Opus 4.7 <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>1.8.0</Version>
+    <Version>1.9.0</Version>
   </PropertyGroup>
 
   <ItemGroup>