Add File -> Minimize to tray toggle (default on)

Adds a checkable MenuItem so the user can opt out of the hide-to-tray
behavior. Persisted per-user to %APPDATA%\WebhookServer\gui.json so the
choice survives restarts.

When ticked (default): X / Alt+F4 / minimize hide to tray, GUI process
keeps running, tray icon persists.

When unticked: X actually closes the app, minimize is a regular
Windows minimize.

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

src/WebhookServer.Gui/MainWindow.xaml.cs

@@ -38,7 +38,7 @@ public partial class MainWindow : Window
 
     private void OnClosing(object? sender, CancelEventArgs e)
     {
-        if (ExitForReal)
+        if (ExitForReal || !_vm.MinimizeToTrayEnabled)
         {
             _tray.Dispose();
             return;
@@ -58,9 +58,10 @@ public partial class MainWindow : Window
 
     private void OnStateChanged(object? sender, EventArgs e)
     {
-        // Minimize-to-tray: hide the window when the user minimizes; restoring is
-        // via the tray icon's double-click or context menu.
-        if (WindowState == WindowState.Minimized)
+        // Minimize-to-tray: hide the window when the user minimizes IF they've
+        // opted in via File -> Minimize to tray. Otherwise behave like a normal
+        // Windows minimize.
+        if (WindowState == WindowState.Minimized && _vm.MinimizeToTrayEnabled)
         {
             Hide();
             ShowInTaskbar = false;