Fix macOS quit: intercept applicationShouldTerminate: to allow real exit

GLFW's applicationShouldTerminate: converts all quit paths (Cmd+Q, dock,
menu bar) into window close requests and returns NSTerminateCancel, so our
close-to-hide intercept was hiding the window instead of letting the app exit.

Fix by implementing applicationShouldTerminate: in ReopenDelegate, setting a
darwinQuitting flag before forwarding to GLFW's delegate. The close intercept
checks this flag and calls Window.Close() instead of Hide() when quitting.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

ui/app_delegate_darwin.go

@@ -11,6 +11,14 @@ import "fyne.io/fyne/v2"
 
 var darwinAppDelegateReopenWindow fyne.Window
 
+// darwinQuitting is set to true when applicationShouldTerminate: fires so
+// the close intercept knows to actually close instead of hide.
+var darwinQuitting bool
+
+func isRealQuit() bool {
+	return darwinQuitting
+}
+
 func installReopenHandler(w fyne.Window) {
 	darwinAppDelegateReopenWindow = w
 	C.installReopenDelegate()
@@ -25,3 +33,8 @@ func appReopened() {
 		fyne.Do(darwinAppDelegateReopenWindow.Show)
 	}()
 }
+
+//export appShouldTerminate
+func appShouldTerminate() {
+	darwinQuitting = true
+}