feat: standalone buttons for all plugin pages + window system for modals

Standalone buttons:
- Add ExternalLink button to Dms, Calendar, Mail, ContactsList toolbars
- Create standalone pages for each (no sidebar, full screen)
- Add standalone routes outside ProtectedRoute

Window system for modals:
- AppointmentModal -> AppointmentEditForm + openWindow()
- ComposeModal -> MailComposeForm + openWindow()
- FilePreviewModal -> FilePreviewContent + openWindow()
- Calendar, Mail, Dms use openWindow() instead of modal state

frontend/src/pages/MailStandalone.tsx

@@ -0,0 +1,10 @@
+import React from 'react';
+import { MailPage } from './Mail';
+
+export function MailStandalonePage() {
+  return (
+    <div className="h-screen w-screen overflow-hidden bg-white">
+      <MailPage />
+    </div>
+  );
+}