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/AboutDialog.cs

@@ -20,6 +20,28 @@ internal sealed class AboutDialog : Form
     /// updates" path.</summary>
     private const string ReleaseNotes =
         """
+        RemSound v1.9
+
+        Critical auto-updater fix. No wire-format or
+        audio-pipeline changes — v1.5 through v1.9 peers
+        interoperate.
+
+        Bug fix:
+          * The auto-updater now actually installs updates.
+            Every previous version had a fault in the update
+            step: the folder path handed to the file-copy
+            command ended in a backslash, which Windows
+            mis-read, so the copy was rejected and no files
+            were ever replaced. Check for updates would
+            download the new version but never apply it. That
+            copy step is fixed.
+
+        Because the fault was in the OLD version doing the
+        updating, v1.8 and earlier cannot install v1.9 for you
+        — install v1.9 by hand once (download the zip, extract
+        it over your RemSound folder). From v1.9 onward, Check
+        for updates installs every update automatically.
+
         RemSound v1.8
 
         Updater polish and a rewritten user manual. No

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>

src/RemSound.App/RemSoundUpdater.cs

@@ -232,6 +232,15 @@ internal sealed class RemSoundUpdater : IDisposable
         var failureMarker = Path.Combine(installDir, "update-failed.txt");
         var stagingDir = Path.Combine(installDir, "_update");
         var remsoundExe = Path.Combine(installDir, "RemSound.exe");
+        // Robocopy source/destination, with any trailing directory separator stripped.
+        // CRITICAL BUG FIX (2026-05-19): installDir is AppContext.BaseDirectory, which ends
+        // in a backslash. A quoted path that ends in a backslash — "D:\dir\" — is mis-parsed
+        // on the command line: the \" is read as an ESCAPED quote, so robocopy never receives
+        // a valid destination argument and exits immediately with code 16 (no files copied).
+        // That silently broke EVERY auto-update in every release to date. The robocopy line
+        // below MUST use these trimmed forms, never the raw {installDir} / {stagingRoot}.
+        var stagingArg = stagingRoot.TrimEnd('\\', '/');
+        var installArg = installDir.TrimEnd('\\', '/');
         return $"""
         @echo off
         setlocal
@@ -256,7 +265,7 @@ internal sealed class RemSoundUpdater : IDisposable
         rem startup settings) and their data folders (logs / profiles / recordings). An update
         rem replaces APP files only. build-release.ps1 already keeps those out of the release
         rem zip; this is the second line of defence so a bad zip still can't clobber them.
-        robocopy "{stagingRoot}" "{installDir}" /E /IS /IT /NFL /NDL /NJH /NJS /R:60 /W:1 /XF _apply-update.cmd /XF _update-helper.log /XF update-failed.txt /XF remsound.config.json /XD logs profiles recordings _update /LOG+:"%LOG%"
+        robocopy "{stagingArg}" "{installArg}" /E /IS /IT /NFL /NDL /NJH /NJS /R:60 /W:1 /XF _apply-update.cmd /XF _update-helper.log /XF update-failed.txt /XF remsound.config.json /XD logs profiles recordings _update /LOG+:"%LOG%"
         set "ROBO_EXIT=%ERRORLEVEL%"
         echo %DATE% %TIME% robocopy exit=%ROBO_EXIT% >> "%LOG%"