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