Release LevelGleichBorder 1.5

• Restrict /challenge commands to OP players and console
• Fix reset persistence by clearing saved player contribution data
• Improve console handling for challenge commands
• Update plugin metadata to version 1.5
• Update README

• Cooperative Growth: The border radius is calculated by: Initial Radius (1.0) + Sum of all level-ups from all players.
• Rewarding Progression: Every 10 individual level-ups, a player is rewarded with End City Treasure loot directly into their inventory.
• Balancing Now border grows on every new player level, not only new max levels reached.