Multi-Platform Reservation & Chat App

I'm in need of an application that allows users to preview menu options, reserve slots via a calendar-based system, chat with others, and make secure payments. The key features I need include:

- **Cross-Platform Support:** The application must be compatible with both mobile phones and desktop computers.
- **Design Preferences:** I'd like the app to have a modern and minimalist design.
- **Calendar-Based Reservations:** Users should be able to book slots through a calendar interface.
- **Chat Functionality:** The app should have a chat feature that can align with user preferences.
- **Payment Gateways:** It's crucial that the app integrates with PayPal, Stripe, and Square for secure and diverse payment options.

The ideal candidate for this job should have experience in developing multi-platform applications with the above-mentioned features. Proven proficiency in integrating multiple payment gateways and chat functionalities is highly desirable. A solid understanding of calendar-based systems will be an added advantage. A portfolio showcasing similar projects would be greatly appreciated.