Release v3.6: rebuild the self-updater (in-app, rollback-safe); warnings to front

Replace the generated cmd.exe + robocopy update helper — which silently
failed on some machines — with an in-app C# installer:
  * Stage the new version to a per-user temp folder off the install and run
    the new RemSound.exe from there, so nothing in the install is locked by
    the updater itself.
  * Wait for the old process to fully exit (real WaitForExit), then
    back-up-and-swap files in C# with retry + rename-aside; roll the install
    back to the previous version on any failure, so a failed update can never
    leave a half-installed RemSound.
  * Log every step to updater.log; clean up old stages and legacy batch
    artefacts on launch. Removed the old BuildInstallScript batch generator.

Route the "RemSound is already running" dialog and its follow-up message
through ForegroundDialog so they surface in front from a background-relaunched
copy, matching the earlier post-update fix.

Docs: rewrite the readme update sections for the new mechanism, regenerate
MANUAL.md, refresh the About-box changelog and RELEASE_NOTES.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/RemSound.App/ForegroundDialog.cs

@@ -39,13 +39,23 @@ internal static class ForegroundDialog
         Show<object?>(owner => { show(owner); return null; });
 
     /// <summary>Force <paramref name="hWnd"/> to the foreground even when RemSound isn't the active
-    /// app. A plain SetForegroundWindow from a background process is refused by Windows; attaching
-    /// our input queue to the current foreground thread for the call lifts that restriction. The
-    /// owner is top-most regardless, so this is belt-and-braces for focus.</summary>
+    /// app — including the hardest case: a process the auto-updater RELAUNCHED in the background while
+    /// RemSound was minimised. Windows refuses SetForegroundWindow to a background-spawned process (it
+    /// only flashes the taskbar button — invisible to a screen-reader user), so we (1) drop the system
+    /// foreground-lock timeout to 0 for the duration of the call, and (2) attach our input queue to the
+    /// current foreground thread. Both the timeout and the attachment are restored immediately after.</summary>
     private static void ForceForeground(IntPtr hWnd)
     {
+        uint savedTimeout = 0;
+        var loweredTimeout = false;
         try
         {
+            // Lift Windows' foreground-stealing lock for this one call — the key fix for a dialog that
+            // must surface from a background-relaunched (post-update) process, where AttachThreadInput
+            // alone isn't enough and the dialog would otherwise open behind everything.
+            if (SystemParametersInfoGet(SPI_GETFOREGROUNDLOCKTIMEOUT, 0, out savedTimeout, 0))
+                loweredTimeout = SystemParametersInfoSet(SPI_SETFOREGROUNDLOCKTIMEOUT, 0, IntPtr.Zero, SPIF_SENDCHANGE);
+
             var foreThread = GetWindowThreadProcessId(GetForegroundWindow(), out _);
             var thisThread = GetCurrentThreadId();
             if (foreThread != 0 && foreThread != thisThread)
@@ -65,12 +75,26 @@ internal static class ForegroundDialog
             }
         }
         catch { /* best-effort; the owner is top-most anyway */ }
+        finally
+        {
+            if (loweredTimeout)
+            {
+                try { SystemParametersInfoSet(SPI_SETFOREGROUNDLOCKTIMEOUT, 0, new IntPtr((long)savedTimeout), SPIF_SENDCHANGE); }
+                catch { /* restoring the user's setting is best-effort */ }
+            }
+        }
     }
 
+    private const uint SPI_GETFOREGROUNDLOCKTIMEOUT = 0x2000;
+    private const uint SPI_SETFOREGROUNDLOCKTIMEOUT = 0x2001;
+    private const uint SPIF_SENDCHANGE = 0x0002;
+
     [DllImport("user32.dll")] private static extern IntPtr GetForegroundWindow();
     [DllImport("user32.dll")] private static extern uint GetWindowThreadProcessId(IntPtr hWnd, out uint pid);
     [DllImport("user32.dll")] private static extern uint GetCurrentThreadId();
     [DllImport("user32.dll")] private static extern bool AttachThreadInput(uint idAttach, uint idAttachTo, bool fAttach);
     [DllImport("user32.dll")] private static extern bool SetForegroundWindow(IntPtr hWnd);
     [DllImport("user32.dll")] private static extern bool BringWindowToTop(IntPtr hWnd);
+    [DllImport("user32.dll", EntryPoint = "SystemParametersInfoW")] private static extern bool SystemParametersInfoGet(uint uiAction, uint uiParam, out uint pvParam, uint fWinIni);
+    [DllImport("user32.dll", EntryPoint = "SystemParametersInfoW")] private static extern bool SystemParametersInfoSet(uint uiAction, uint uiParam, IntPtr pvParam, uint fWinIni);
 }