Implement JSON message format for chat messaging

[Copilot]

This PR implements a structured JSON message format for the DrizLink P2P chat application, replacing the previous plain text messaging system.

Changes Made

Enhanced Message Structure

• Added JSON serialization tags to the Message struct in server/interfaces/interfaces.go
• Introduced a new Type field to distinguish between message types ("chat", "system", "command")
• Added ToJSON() and MessageFromJSON() helper methods for easy serialization/deserialization

Server-Side Improvements

• Modified BroadcastMessage() function to send JSON-formatted chat messages with timestamps
• Created BroadcastSystemMessage() function specifically for system notifications (user join/leave events)
• All messages now include structured metadata: sender ID, username, content, timestamp, and type

Client-Side Enhancements

• Updated ReadLoop() function to parse incoming JSON messages
• Maintained backward compatibility with legacy plain text messages
• Improved message display formatting with timestamps and proper styling

Message Format Example

Before (Plain Text):

Alice: Hello everyone!

After (JSON):

{
  "senderId": "user123",
  "senderUsername": "Alice",
  "content": "Hello everyone!",
  "timestamp": "2025-07-30 14:56:52",
  "type": "chat"
}

Benefits

• Structured Data: Clear field separation for programmatic processing
• Timestamps: All messages now include creation timestamps
• Message Types: Distinction between chat messages and system notifications
• Metadata: Enhanced sender identification and message context
• Backward Compatibility: Graceful fallback to handle legacy plain text messages
• Future-Proof: Extensible format for additional features

Testing

The implementation has been thoroughly tested with:

• JSON serialization/deserialization verification
• Message type handling for both chat and system messages
• Backward compatibility with existing plain text format
• Build verification for both server and client components

This change provides a solid foundation for future enhancements while maintaining compatibility with existing functionality.

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