safeserver

Fix coordinates leak vulnerability during authentication

• Enhance player security by teleporting authenticating players to a safe, calculated location at (0,Y,0) instead of keeping them at their join position
• Calculate Y coordinate based on the actual surface height at (0,0) to ensure players are safely above ground
• Detect and remedy situations where a player disconnects during authentication and rejoins, preventing them from being stuck in spectator mode
• Update README to clearly document this security feature

Resolves #5: Coordinates Weakness vulnerability