Single-instance launcher + tolerant build pipeline

Single-instance
---------------
Two PSLauncher.exe instances ran in parallel during the user's last
release build, locking the repo-root copy and breaking the publish.
Two-layer protection:

1. App.xaml.cs uses a Global\ named Mutex (UUID-based key) created at
   OnStartup. If the mutex is already held, the second instance
   PostMessages a registered Windows message (PSLAUNCHER_BRING_TO_FRONT)
   to HWND_BROADCAST and Shutdown()s immediately.

2. MainWindow hooks WndProc via SourceInitialized + HwndSource.AddHook;
   when it sees the broadcast, it restores from minimized, calls
   ShowWindow(SW_RESTORE) + Activate() + SetForegroundWindow so the
   already-running instance pops to the user's foreground.

Net result: clicking the launcher icon a second time pops the existing
window instead of starting a duplicate process.

Tolerant build pipeline
-----------------------
- Both csproj post-publish copy targets now have
  ContinueOnError="WarnAndContinue". A locked PSLauncher.exe at the
  repo root no longer fails the entire publish — the binary still
  exists in bin\Release\...\publish\ and the user gets a warning
  instead of an error.

- All four .bat scripts (build-launcher / build-updater / build-all /
  build-installer) now run `taskkill /F /IM PSLauncher.exe /T` and the
  same for PSLauncher.Updater.exe before the publish step. This
  defensively closes any stray instance left behind by previous tests
  so the build pipeline is clean.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/PSLauncher.Updater/PSLauncher.Updater.csproj

@@ -32,7 +32,8 @@
     <Copy SourceFiles="$(PublishDir)$(AssemblyName).exe"
           DestinationFolder="$(RepoRoot)"
           SkipUnchangedFiles="false"
-          OverwriteReadOnlyFiles="true" />
+          OverwriteReadOnlyFiles="true"
+          ContinueOnError="WarnAndContinue" />
     <Message Importance="high" Text="Copied $(AssemblyName).exe to $(RepoRoot)" />
   </Target>