macos: quit if we have no visible windows

macos/Sources/SettingsView.swift

@@ -1,6 +1,9 @@
 import SwiftUI
 
 struct SettingsView: View {
+    // We need access to our app delegate to know if we're quitting or not.
+    @EnvironmentObject private var appDelegate: AppDelegate
+    
     var body: some View {
         HStack {
             Image("AppIconImage")
@@ -18,6 +21,10 @@ struct SettingsView: View {
         }
         .padding()
         .frame(minWidth: 500, maxWidth: 500, minHeight: 156, maxHeight: 156)
+        .onChange(of: appDelegate.confirmQuit) { value in
+            guard value else { return }
+            NSApplication.shared.reply(toApplicationShouldTerminate: true)
+        }
     }
 }