Adds a dialog to test email settings and integrates with the `/api/settings/test/email` API endpoint.
- Added a "Test Email" button to the Mail Settings tab.
- Implemented a dialog for entering the recipient email and selecting a template.
- Added UI elements for the dialog (input fields, radio buttons, and a send button).
- Updated `src/services/settingsService.ts` to include the test email API call.
- Added UI components for the dialog (using existing UI components).
- Updated `src/components/settings/general/MailSettingsTab.tsx` to include the test email functionality.
Tola Leng