Convert Local HTML Web Application to Real-Time Multi-User Collaborative Platform

I have developed a web application in HTML that currently works locally on a single computer. I need to transform this application into a live, online platform where multiple users can access it simultaneously from different computers and collaborate in real-time.

Current Situation:

Existing HTML web application (fully functional locally)
Works on single PC only
Need to maintain the exact same appearance and functionality
Requirements:

Online Hosting & Deployment:
Deploy the application to a web server
Ensure it's accessible via internet URL
Maintain current design and functionality exactly as is
Real-Time Collaboration Features:
Multiple users can access simultaneously from different computers
Real-time synchronization of all user actions
Live updates visible to all connected users instantly
Conflict resolution for simultaneous edits
Technical Implementation:
Convert static HTML to dynamic web application
Implement WebSocket or similar technology for real-time updates
Database integration for data persistence
Session management for multiple concurrent users
User Management:
Basic user identification system
Show active users currently online
Handle user connections/disconnections gracefully
Technical Stack Preferences:

Backend: Node.js, Python (Django/Flask), or PHP
Real-time: WebSocket, Socket.IO, or Server-Sent Events
Database: MySQL, PostgreSQL, or MongoDB
Frontend: Keep existing HTML/CSS/JavaScript structure
Deliverables:

Fully deployed online application
Real-time multi-user functionality
Source code with documentation
Basic setup/deployment instructions
Testing on multiple devices/browsers
Additional Notes:

The current application interface must remain unchanged
Performance should handle at least 10-20 concurrent users
Basic security measures for online deployment
Cross-browser compatibility required