Fix multiplayer synchronization, chat broadcasting, and combat system

[Copilot]

This pull request introduces major improvements to both the project documentation and the frontend visual experience, as well as some optimizations and debugging enhancements in the client-side JavaScript. The most important changes are the addition of a comprehensive Copilot instructions file, a complete overhaul of the README.md to focus on the pure Java networking backend, significant upgrades to the CSS for a more modern and dynamic look, and client-side performance and debugging improvements.

Documentation and Project Guidance

• Added .github/copilot-instructions.md with detailed project goals, Java networking concepts, implementation guidelines, folder structure, and best practices for Copilot to guide future development and AI code generation.
• Completely rewrote README.md to focus on the pure Java 21 multiplayer server, outlining networking architecture (TCP, UDP, NIO), game features, learning objectives, and setup instructions.

Frontend Visual and UX Enhancements

• Upgraded client/css/style.css to include advanced background gradients, glowing and animated text effects, improved container styling, and backdrop blur for a more modern, immersive look. [1] [2] [3] [4]

Client-Side Performance and Debugging

• Optimized movement update logic in client/js/game.js by introducing throttling and change thresholds to reduce network traffic, and added properties for more efficient rendering.
• Enhanced debugging by adding console.log statements for WebSocket connection events and message handling, making it easier to diagnose connectivity issues. [1] [2]