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.App/App.xaml.cs

@@ -1,6 +1,7 @@
 using System.IO;
 using System.Net.Http;
 using System.Net.Http.Headers;
+using System.Runtime.InteropServices;
 using System.Windows;
 using Microsoft.Extensions.DependencyInjection;
 using Microsoft.Extensions.Hosting;
@@ -24,6 +25,20 @@ namespace PSLauncher.App;
 public partial class App : Application
 {
     private IHost? _host;
+    private static System.Threading.Mutex? _singleInstanceMutex;
+
+    private const string SingleInstanceMutexName = "Global\\PSLauncher-3F8E2C1A-9B47-4D5E-A0F8-2E9D1B6C7A3F";
+    private const string BringToFrontMessage = "PSLAUNCHER_BRING_TO_FRONT";
+    private static readonly int WM_PSLAUNCHER_BRING = RegisterWindowMessage(BringToFrontMessage);
+
+    [DllImport("user32.dll")]
+    private static extern int RegisterWindowMessage(string lpString);
+
+    [DllImport("user32.dll")]
+    [return: MarshalAs(UnmanagedType.Bool)]
+    private static extern bool PostMessage(IntPtr hWnd, int Msg, IntPtr wParam, IntPtr lParam);
+
+    private const int HWND_BROADCAST = 0xFFFF;
 
     public static string LogsDirectory { get; private set; } = string.Empty;
 
@@ -31,6 +46,17 @@ public partial class App : Application
     {
         base.OnStartup(e);
 
+        // Single-instance : si une autre instance tourne déjà, on lui demande de
+        // se mettre au premier plan via un broadcast Windows et on quitte tout de suite.
+        _singleInstanceMutex = new System.Threading.Mutex(initiallyOwned: true,
+            name: SingleInstanceMutexName, out var createdNew);
+        if (!createdNew)
+        {
+            PostMessage((IntPtr)HWND_BROADCAST, WM_PSLAUNCHER_BRING, IntPtr.Zero, IntPtr.Zero);
+            Shutdown();
+            return;
+        }
+
         // Logs avant tout — pour ne rien perdre des erreurs au boot
         LogsDirectory = Path.Combine(
             Environment.GetFolderPath(Environment.SpecialFolder.LocalApplicationData),